74 CHEMISTRY
The Periodic Table is arguably the most important concept in
chemistry, both in principle and in practice. It is the everyday
support for students, it suggests new avenues of research to
professionals, and it provides a succinct organization of the
whole of chemistry. It is a remarkable demonstration of the
fact that the chemical elements are not a random cluster of
entities but instead display trends and lie together in families.
An awareness of the Periodic Table is essential to anyone who
wishes to disentangle the world and see how it is built up
from the fundamental building blocks of the chemistry, the
chemical elements.
Glenn T. Seaborg
In this Unit, we will study the historical development of the
Periodic Table as it stands today and the Modern Periodic
Law. We will also learn how the periodic classification
follows as a logical consequence of the electronic
configuration of atoms. Finally, we shall examine some of
the periodic trends in the physical and chemical properties
of the elements.
3.1 WHY DO WE NEED TO CLASSIFY ELEMENTS ?
We know by now that the elements are the basic units of all
types of matter. In 1800, only 31 elements were known. By
1865, the number of identified elements had more than
doubled to 63. At present 114 elements are known. Of
them, the recently discovered elements are man-made.
Efforts to synthesise new elements are continuing. With
such a large number of elements it is very difficult to study
individually the chemistry of all these elements and their
innumerable compounds individually. To ease out this
problem, scientists searched for a systematic way to
organise their knowledge by classifying the elements. Not
only that it would rationalize known chemical facts about
elements, but even predict new ones for undertaking further
study.
UNIT 3
After studying this Unit, you will be
able to
•
appreciate how the concept of
grouping elements in accordance to
their properties led to the
development of Periodic Table.
•
understand the Periodic Law;
•
understand the significance of
atomic number and electronic
configuration as the basis for
periodic classification;
•
name the elements with
Z >100 according to IUPAC
nomenclature;
•
classify elements into s, p, d, f
blocks and learn their main
characteristics;
•
recognise the periodic trends in
physical and chemical properties of
elements;
•
compare the reactivity of elements
and correlate it with their
occurrence in nature;
•
explain the relationship between
ionization enthalpy and metallic
character;
•
use scientific vocabulary
appropriately to communicate ideas
related to certain important
properties of atoms e.g., atomic/
ionic radii, ionization enthalpy,
electron gain enthalpy,
electronegativity, valence of
elements.
CLASSIFICATION OF ELEMENTS AND
PERIODICITY IN PROPERTIES
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75CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
3.2 GENESIS OF PERIODIC
CLASSIFICATION
Classification of elements into groups and
development of Periodic Law and Periodic
Table are the consequences of systematising
the knowledge gained by a number of scientists
through their observations and experiments.
The German chemist, Johann Dobereiner in
early 1800’s was the first to consider the idea
of trends among properties of elements. By
1829 he noted a similarity among the physical
and chemical properties of several groups of
three elements (Triads). In each case, he
noticed that the middle element of each of the
Triads had an atomic weight about half way
between the atomic weights of the other two
(Table 3.1). Also the properties of the middle
element were in between those of the other
two members. Since Dobereiner’s relationship,
referred to as the Law of Triads, seemed to
work only for a few elements, it was dismissed
as coincidence. The next reported attempt to
classify elements was made by a French
geologist, A.E.B. de Chancourtois in 1862. He
arranged the then known elements in order of
increasing atomic weights and made a
cylindrical table of elements to display the
periodic recurrence of properties. This also did
not attract much attention. The English
chemist, John Alexander Newlands in 1865
profounded the Law of Octaves. He arranged
the elements in increasing order of their atomic
weights and noted that every eighth element
had properties similar to the first element
(Table 3.2). The relationship was just like every
eighth note that resembles the first in octaves
of music. Newlands’s Law of Octaves seemed
to be true only for elements up to calcium.
Although his idea was not widely accepted at
that time, he, for his work, was later awarded
Davy Medal in 1887 by the Royal Society,
London.
The Periodic Law, as we know it today owes
its development to the Russian chemist, Dmitri
Mendeleev (1834-1907) and the German
chemist, Lothar Meyer (1830-1895). Working
independently, both the chemists in 1869
proposed that on arranging elements in the
increasing order of their atomic weights,
similarities appear in physical and chemical
properties at regular intervals. Lothar Meyer
plotted the physical properties such as atomic
volume, melting point and boiling point
against atomic weight and obtained a
periodically repeated pattern. Unlike
Newlands, Lothar Meyer observed a change in
length of that repeating pattern. By 1868,
Lothar Meyer had developed a table of the
Element Atomic Element Atomic Element Atomic
weight weight weight
Li 7 Ca 40 Cl 35.5
Na 23 Sr 88 Br 80
K 39 Ba 137 I 127
Table 3.1 Dobereiner’s Triads
Table 3.2 Newlands’ Octaves
Element Li Be B C N O F
At. wt. 7 9 11
12 14 16 19
Element Na Mg Al Si P S Cl
At. wt. 23 24 27 29 31 32 35.5
Element K Ca
At. wt. 39 40
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76 CHEMISTRY
elements that closely resembles the Modern
Periodic Table. However, his work was not
published until after the work of Dmitri
Mendeleev, the scientist who is generally
credited with the development of the Modern
Periodic Table.
While Dobereiner initiated the study of
periodic relationship, it was Mendeleev who
was responsible for publishing the Periodic
Law for the first time. It states as follows :
The properties of the elements are a
periodic function of their atomic
weights.
Mendeleev arranged elements in horizontal
rows and vertical columns of a table in order
of their increasing atomic weights in such a
way that the elements with similar properties
occupied the same vertical column or group.
Mendeleev’s system of classifying elements was
more elaborate than that of Lothar Meyer’s.
He fully recognized the significance of
periodicity and used broader range of physical
and chemical properties to classify the
elements. In particular, Mendeleev relied on
the similarities in the empirical formulas and
properties of the compounds formed by the
elements. He realized that some of the elements
did not fit in with his scheme of classification
if the order of atomic weight was strictly
followed. He ignored the order of atomic
weights, thinking that the atomic
measurements might be incorrect, and placed
the elements with similar properties together.
For example, iodine with lower atomic weight
than that of tellurium (Group VI) was placed
in Group VII along with fluorine, chlorine,
bromine because of similarities in properties
(Fig. 3.1). At the same time, keeping his
primary aim of arranging the elements of
similar properties in the same group, he
proposed that some of the elements were still
undiscovered and, therefore, left several gaps
in the table. For example, both gallium and
germanium were unknown at the time
Mendeleev published his Periodic Table. He left
the gap under aluminium and a gap under
silicon, and called these elements Eka-
Aluminium and Eka-Silicon. Mendeleev
predicted not only the existence of gallium and
germanium, but also described some of their
general physical properties. These elements
were discovered later. Some of the properties
predicted by Mendeleev for these elements and
those found experimentally are listed in
Table 3.3.
The boldness of Mendeleev’s quantitative
predictions and their eventual success made
him and his Periodic Table famous.
Mendeleev’s Periodic Table published in 1905
is shown in Fig. 3.1.
Property Eka-aluminium Gallium Eka-silicon Germanium
(predicted) (found) (predicted) (found)
Atomic weight 68 70 72 72.6
Density / (g/cm
3
) 5.9 5.94 5.5 5.36
Melting point /K Low 302.93 High 1231
Formula of oxide E
2
O
3
Ga
2
O
3
EO
2
GeO
2
Formula of chloride ECl
3
GaCl
3
ECl
4
GeCl
4
Table 3.3 Mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and
Eka-silicon (Germanium)
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77CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
PERIODIC SYSTEM OF THE ELEMENTS IN GROUPS AND SERIES
Fig. 3.1 Mendeleev’s Periodic Table published earlier
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78 CHEMISTRY
3.3 MODERN PERIODIC LAW AND THE
PRESENT FORM OF THE PERIODIC
TABLE
We must bear in mind that when Mendeleev
developed his Periodic Table, chemists knew
nothing about the internal structure of atom.
However, the beginning of the 20
th
century
witnessed profound developments in theories
about sub-atomic particles. In 1913, the
English physicist, Henry Moseley observed
regularities in the characteristic X-ray spectra
of the elements. A plot of
ν
(where
ν
is
frequency of X-rays emitted) against atomic
number (Z ) gave a straight line and not the
plot of
ν
vs atomic mass. He thereby showed
that the atomic number is a more fundamental
property of an element than its atomic mass.
Mendeleev’s Periodic Law was, therefore,
accordingly modified. This is known as the
Modern Periodic Law and can be stated as :
The physical and chemical properties
of the elements are periodic functions
of their atomic numbers.
The Periodic Law revealed important
analogies among the 94 naturally occurring
elements (neptunium and plutonium like
actinium and protoactinium are also found in
pitch blende – an ore of uranium). It stimulated
renewed interest in Inorganic Chemistry and
has carried into the present with the creation
of artificially produced short-lived elements.
You may recall that the atomic number is
equal to the nuclear charge (i.e., number of
protons) or the number of electrons in a neutral
atom. It is then easy to visualize the significance
of quantum numbers and electronic
configurations in periodicity of elements. In
fact, it is now recognized that the Periodic Law
is essentially the consequence of the periodic
variation in electronic configurations, which
indeed determine the physical and chemical
properties of elements and their compounds.
Numerous forms of Periodic Table have
been devised from time to time. Some forms
emphasise chemical reactions and valence,
whereas others stress the electronic
configuration of elements. A modern version,
the so-called “long form” of the Periodic Table
of the elements (Fig. 3.2), is the most convenient
and widely used. The horizontal rows (which
Mendeleev called series) are called periods and
the vertical columns, groups. Elements having
similar outer electronic configurations in their
atoms are arranged in vertical columns,
referred to as groups or families. According
to the recommendation of International Union
of Pure and Applied Chemistry (IUPAC), the
groups are numbered from 1 to 18 replacing
the older notation of groups IA … VIIA, VIII, IB
… VIIB and 0.
There are altogether seven periods. The
period number corresponds to the highest
principal quantum number (n) of the elements
in the period. The first period contains 2
elements. The subsequent periods consists of
8, 8, 18, 18 and 32 elements, respectively. The
seventh period is incomplete and like the sixth
period would have a theoretical maximum (on
the basis of quantum numbers) of 32 elements.
In this form of the Periodic Table, 14 elements
of both sixth and seventh periods (lanthanoids
and actinoids, respectively) are placed in
separate panels at the bottom
*
.
3.4 NOMENCLATURE OF ELEMENTS WITH
ATOMIC NUMBERS > 100
The naming of the new elements had been
traditionally the privilege of the discoverer (or
discoverers) and the suggested name was
ratified by the IUPAC. In recent years this has
led to some controversy. The new elements with
very high atomic numbers are so unstable that
only minute quantities, sometimes only a few
atoms of them are obtained. Their synthesis
and characterisation, therefore, require highly
* Glenn T. Seaborg’s work in the middle of the 20
th
century starting with the discovery of plutonium in 1940, followed by
those of all the transuranium elements from 94 to 102 led to reconfiguration of the periodic table placing the actinoids
below the lanthanoids. In 1951, Seaborg was awarded the Nobel Prize in chemistry for his work. Element 106 has been
named Seaborgium (Sg) in his honour.
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79CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
Fig. 3.2 Long form of the Periodic Table of the Elements with their atomic numbers and ground state outer
electronic configurations. The groups are numbered 1-18 in accordance with the 1984 IUPAC
recommendations. This notation replaces the old numbering scheme of IA–VIIA, VIII, IB–VIIB and 0 for
the elements.
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80 CHEMISTRY
sophisticated costly equipment and laboratory.
Such work is carried out with competitive spirit
only in some laboratories in the world.
Scientists, before collecting the reliable data on
the new element, at times get tempted to claim
for its discovery. For example, both American
and Soviet scientists claimed credit for
discovering element 104. The Americans
named it Rutherfordium whereas Soviets
named it Kurchatovium. To avoid such
problems, the IUPAC has made
recommendation that until a new element’s
discovery is proved, and its name is officially
recognised, a systematic nomenclature be
derived directly from the atomic number of the
element using the numerical roots for 0 and
numbers 1-9. These are shown in Table 3.4.
The roots are put together in order of digits
Atomic Name according to Symbol IUPAC IUPAC
Number IUPAC nomenclature Official Name Symbol
101 Unnilunium Unu Mendelevium Md
102 Unnilbium Unb Nobelium No
103 Unniltrium Unt Lawrencium Lr
104 Unnilquadium Unq Rutherfordium Rf
105 Unnilpentium Unp Dubnium Db
106 Unnilhexium Unh Seaborgium Sg
107 Unnilseptium Uns Bohrium Bh
108 Unniloctium Uno Hassium Hs
109 Unnilennium Une Meitnerium Mt
110 Ununnillium Uun Darmstadtium Ds
111 Unununnium Uuu Rontgenium Rg
112 Ununbium Uub Copernicium Cn
113 Ununtrium Uut Nihonium Nh
114 Ununquadium Uuq Flerovium Fl
115 Ununpentium Uup Moscovium Mc
116 Ununhexium Uuh Livermorium Lv
117 Ununseptium Uus Tennessine Ts
118 Ununoctium Uuo Oganesson Og
–
Table 3.5 Nomenclature of Elements with Atomic Number Above 100
Table 3.4 Notation for IUPAC Nomenclature
of Elements
which make up the atomic number and “ium”
is added at the end. The IUPAC names for
elements with Z above 100 are shown in
Table 3.5.
Digit Name Abbreviation
0 nil n
1 un u
2 bi b
3 tri t
4 quad q
5 pent p
6 hex h
7 sept s
8 oct o
9 enn e
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81CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
Thus, the new element first gets a
temporary name, with symbol consisting of
three letters. Later permanent name and
symbol are given by a vote of IUPAC
representatives from each country. The
permanent name might reflect the country (or
state of the country) in which the element was
discovered, or pay tribute to a notable
scientist. As of now, elements with atomic
numbers up to 118 have been discovered.
Official names of all elements have been
announced by IUPAC.
Problem 3.1
What would be the IUPAC name and
symbol for the element with atomic
number 120?
Solution
From Table 3.4, the roots for 1, 2 and 0
are un, bi and nil, respectively. Hence, the
symbol and the name respectively are Ubn
and unbinilium.
3.5 ELECTRONIC CONFIGURATIONS OF
ELEMENTS AND THE PERIODIC
TABLE
In the preceding unit we have learnt that an
electron in an atom is characterised by a set of
four quantum numbers, and the principal
quantum number (n ) defines the main energy
level known as shell. We have also studied
about the filling of electrons into different
subshells, also referred to as orbitals (s, p, d,
f) in an atom. The distribution of electrons into
orbitals of an atom is called its electronic
configuration. An element’s location in the
Periodic Table reflects the quantum numbers
of the last orbital filled. In this section we will
observe a direct connection between the
electronic configurations of the elements and
the long form of the Periodic Table.
(a) Electronic Configurations in Periods
The period indicates the value of n for the
outermost or valence shell. In other words,
successive period in the Periodic Table is
associated with the filling of the next higher
principal energy level (n = 1, n = 2, etc.). It can
be readily seen that the number of elements in
each period is twice the number of atomic
orbitals available in the energy level that is
being filled. The first period (n = 1) starts with
the filling of the lowest level (1s) and therefore
has two elements — hydrogen (ls
1
) and helium
(ls
2
) when the first shell (
K) is completed. The
second period (n = 2) starts with lithium and
the third electron enters the 2s orbital. The next
element, beryllium has four electrons and has
the electronic configuration 1s
2
2s
2
. Starting
from the next element boron, the 2p orbitals
are filled with electrons when the L shell is
completed at neon (2s
2
2p
6
). Thus there are
8 elements in the second period. The third
period (n = 3) begins at sodium, and the added
electron enters a 3s orbital. Successive filling
of 3s and 3p orbitals gives rise to the third
period of 8 elements from sodium to argon. The
fourth period (n = 4) starts at potassium, and
the added electrons fill up the 4s orbital. Now
you may note that before the 4p orbital is filled,
filling up of 3d orbitals becomes energetically
favourable and we come across the so called
3d transition series of elements. This starts
from scandium (Z = 21) which has the electronic
configuration 3d
1
4s
2
. The 3d orbitals are filled
at zinc (Z=30) with electronic configuration
3d
10
4s
2
. The fourth period ends at krypton
with the filling up of the 4p orbitals. Altogether
we have 18 elements in this fourth period. The
fifth period (n = 5) beginning with rubidium is
similar to the fourth period and contains the
4d transition series starting at yttrium
(Z = 39). This period ends at xenon with the
filling up of the 5p orbitals. The sixth period
(n = 6) contains 32 elements and successive
electrons enter 6s, 4f, 5d and 6p orbitals, in
the order — filling up of the 4f orbitals begins
with cerium (Z = 58) and ends at lutetium
(Z = 71) to give the 4f-inner transition series
which is called the lanthanoid series. The
seventh period (n = 7) is similar to the sixth
period with the successive filling up of the 7s,
5f, 6d and 7p orbitals and includes most of
the man-made radioactive elements. This
period will end at the element with atomic
number 118 which would belong to the noble
gas family. Filling up of the 5f orbitals after
actinium (Z = 89) gives the 5f-inner transition
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82 CHEMISTRY
series known as the actinoid series. The 4f-
and 5f-inner transition series of elements are
placed separately in the Periodic Table to
maintain its structure and to preserve the
principle of classification by keeping elements
with similar properties in a single column.
Problem 3.2
How would you justify the presence of 18
elements in the 5
th
period of the Periodic
Table?
Solution
When n = 5, l = 0, 1, 2, 3. The order in
which the energy of the available orbitals
4d, 5s and 5p increases is 5
s < 4d < 5p.
The total number of orbitals available are
9. The maximum number of electrons that
can be accommodated is 18; and therefore
18 elements are there in the 5
th
period.
(b) Groupwise Electronic Configurations
Elements in the same vertical column or group
have similar valence shell electronic
configurations, the same number of electrons
in the outer orbitals, and similar properties.
For example, the Group 1 elements (alkali
metals) all have ns
1
valence shell electronic
configuration as shown below.
Atomic number Symbol Electronic configuration
3 Li 1s
2
2s
1
(or) [He]2s
1
11 Na 1s
2
2s
2
2p
6
3s
1
(or) [Ne]3s
1
19 K 1s
2
2s
2
2p
6
3s
2
3p
6
4s
1
(or) [Ar]4s
1
37 Rb 1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
6
5s
1
(or) [Kr]5s
1
55 Cs 1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
6
4d
10
5s
2
5p
6
6s
1
(or) [Xe]6s
1
87 Fr [Rn]7s
1
theoretical foundation for the periodic
classification. The elements in a vertical column
of the Periodic Table constitute a group or
family and exhibit similar chemical behaviour.
This similarity arises because these elements
have the same number and same distribution
of electrons in their outermost orbitals. We can
classify the elements into four blocks viz.,
s-block, p-block, d-block and f-block
depending on the type of atomic orbitals that
are being filled with electrons. This is illustrated
in Fig. 3.3. We notice two exceptions to this
categorisation. Strictly, helium belongs to the
s-block but its positioning in the p-block along
with other group 18 elements is justified
because it has a completely filled valence shell
(1s
2
) and as a result, exhibits properties
characteristic of other noble gases. The other
exception is hydrogen. It has only one
s-electron and hence can be placed in group 1
(alkali metals). It can also gain an electron to
achieve a noble gas arrangement and hence it
can behave similar to a group 17 (halogen
family) elements. Because it is a special case,
we shall place hydrogen separately at the top
of the Periodic Table as shown in Fig. 3.2 and
Fig. 3.3. We will briefly discuss the salient
features of the four types of elements marked in
Thus it can be seen that the properties of
an element have periodic dependence upon its
atomic number and not on relative atomic
mass.
3.6 ELECTRONIC CONFIGURATIONS
AND TYPES OF ELEMENTS:
s-, p-, d-, f- BLOCKS
The aufbau (build up) principle and the
electronic configuration of atoms provide a
the Periodic Table. More about these elements
will be discussed later. During the description
of their features certain terminology has been
used which has been classified in section 3.7.
3.6.1 The s-Block Elements
The elements of Group 1 (alkali metals) and
Group 2 (alkaline earth metals) which have ns
1
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83CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
Nh Mc Ts
Og
Fig. 3.3 The types of elements in the Periodic Table based on the orbitals that
are being filled. Also shown is the broad division of elements into METALS
( ) , NON-METALS ( ) and METALLOIDS ( ).
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84 CHEMISTRY
and ns
2
outermost electronic configuration
belong to the s-Block Elements. They are all
reactive metals with low ionization enthalpies.
They lose the outermost electron(s) readily to
form 1+ ion (in the case of alkali metals) or 2+
ion (in the case of alkaline earth metals). The
metallic character and the reactivity increase
as we go down the group. Because of high
reactivity they are never found pure in nature.
The compounds of the s-block elements, with
the exception of those of lithium and beryllium
are predominantly ionic.
3.6.2 The p-Block Elements
The p-Block Elements comprise those
belonging to Group 13 to 18 and these
together with the s-Block Elements are called
the Representative Elements or Main Group
Elements. The outermost electronic
configuration varies from ns
2
np
1
to ns
2
np
6
in
each period. At the end of each period is a noble
gas element with a closed valence shell ns
2
np
6
configuration. All the orbitals in the valence
shell of the
noble gases are completely filled
by electrons and it is very difficult to alter this
stable arrangement by the addition or removal
of electrons. The noble gases thus exhibit very
low chemical reactivity. Preceding the noble gas
family are two chemically important groups of
non-metals. They are the halogens (Group 17)
and the chalcogens (Group 16). These two
groups of elements have highly negative
electron gain enthalpies and readily add one
or two electrons respectively to attain the stable
noble gas configuration. The non-metallic
character increases as we move from left to right
across a period and metallic character increases
as we go down the group.
3.6.3 The d-Block Elements (Transition
Elements)
These are the elements of Group 3 to 12 in the
centre of the Periodic Table. These are
characterised by the filling of inner d orbitals
by electrons and are therefore referred to as
d-Block Elements. These elements have the
general outer electronic configuration
(n-1)d
1-10
ns
0-2
. They are all metals. They mostly
form coloured ions, exhibit variable valence
(oxidation states), paramagnetism and oftenly
used as catalysts. However, Zn, Cd and Hg
which have the electronic configuration,
(n-1) d
10
ns
2
do not show most of the properties
of transition elements. In a way, transition
metals form a bridge between the chemically
active metals of s-block elements and the less
active elements of Groups 13 and 14 and thus
take their familiar name “Transition
Elements”.
3.6.4 The f-Block Elements
(Inner-Transition Elements)
The two rows of elements at the bottom of the
Periodic Table, called the Lanthanoids,
Ce(Z = 58) – Lu(Z = 71) and Actinoids,
Th(Z = 90) – Lr (Z = 103) are characterised by
the outer electronic configuration (n-2)f
1-14
(n-1)d
0–1
ns
2
. The last electron added to each
element is filled in f- orbital. These two series
of elements are hence called the Inner-
Transition Elements (f-Block Elements).
They are all metals. Within each series, the
properties of the elements are quite similar. The
chemistry of the early actinoids is more
complicated than the corresponding
lanthanoids, due to the large number of
oxidation states possible for these actinoid
elements. Actinoid elements are radioactive.
Many of the actinoid elements have been made
only in nanogram quantities or even less by
nuclear reactions and their chemistry is not
fully studied. The elements after uranium are
called Transuranium Elements.
Problem 3.3
The elements Z = 117 and 120 have not
yet been discovered. In which family /
group would you place these elements
and also give the electronic configuration
in each case.
Solution
We see from Fig. 3.2, that element with Z
= 117, would belong to the halogen family
(Group 17) and the electronic
configuration would be [Rn]
5f
14
6d
10
7s
2
7p
5
. The element with Z = 120,
will be placed in Group 2 (alkaline earth
metals), and will have the electronic
configuration [Uuo]8s
2
.
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85CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
3.6.5 Metals, Non-metals and Metalloids
In addition to displaying the classification of
elements into s-, p-, d-, and f-blocks, Fig. 3.3
shows another broad classification of elements
based on their properties. The elements can
be divided into Metals and Non-Metals. Metals
comprise more than 78% of all known elements
and appear on the left side of the Periodic
Table. Metals are usually solids at room
temperature [mercury is an exception; gallium
and caesium also have very low melting points
(303K and 302K, respectively)]. Metals usually
have high melting and boiling points. They are
good conductors of heat and electricity. They
are malleable (can be flattened into thin sheets
by hammering) and ductile (can be drawn into
wires). In contrast, non-metals are located at
the top right hand side of the
Periodic Table.
In fact, in a horizontal row, the property of
elements change from metallic on the left to
non-metallic on the right. Non-metals are
usually solids or gases at room temperature
with low melting and boiling points (boron and
carbon are exceptions). They are poor
conductors of heat and electricity. Most non-
metallic solids are brittle and are neither
malleable nor ductile. The elements become
more metallic as we go down a group; the non-
metallic character increases as one goes from
left to right across the Periodic Table. The
change from metallic to non-metallic character
is not abrupt as shown by the thick zig-zag
line in Fig. 3.3. The elements (e.g., silicon,
germanium, arsenic, antimony and tellurium)
bordering this line and running diagonally
across the Periodic Table show properties that
are characteristic of both metals and non-
metals. These elements are called Semi-metals
or Metalloids.
Problem 3.4
Considering the atomic number and
position in the periodic table, arrange the
following elements in the increasing order
of metallic character : Si, Be, Mg, Na, P.
Solution
Metallic character increases down a group
and decreases along a period as we move
from left to right. Hence the order of
increasing metallic character is: P < Si <
Be < Mg < Na.
3.7 PERIODIC TRENDS IN PROPERTIES
OF ELEMENTS
There are many observable patterns in the
physical and chemical properties of elements
as we descend in a group or move across a
period in the Periodic Table. For example,
within a period, chemical reactivity tends to be
high in Group 1 metals, lower in elements
towards the middle of the table, and increases
to a maximum in the Group 17 non-metals.
Likewise within a group of representative
metals (say alkali metals) reactivity increases
on moving down the group, whereas within a
group of non-metals (say halogens), reactivity
decreases down the group. But why do the
properties of elements follow these trends? And
how can we explain periodicity? To answer
these questions, we must look into the theories
of atomic structure and properties of the atom.
In this section we shall discuss the periodic
trends in certain physical and chemical
properties and try to explain them in terms of
number of electrons and energy levels.
3.7.1 Trends in Physical Properties
There are numerous physical properties of
elements such as melting and boiling points,
heats of fusion and vaporization, energy of
atomization, etc. which show periodic
variations. However, we shall discuss the
periodic trends with respect to atomic and ionic
radii, ionization enthalpy, electron gain
enthalpy and electronegativity.
(a) Atomic Radius
You can very well imagine that finding the size
of an atom is a lot more complicated than
measuring the radius of a ball. Do you know
why? Firstly, because the size of an atom
(~ 1.2 Å i.e., 1.2 × 10
–10
m in radius) is very
small. Secondly, since the electron cloud
surrounding the atom does not have a sharp
boundary, the determination of the atomic size
cannot be precise. In other words, there is no
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86 CHEMISTRY
practical way by which the size of an individual
atom can be measured. However, an estimate
of the atomic size can be made by knowing the
distance between the atoms in the combined
state. One practical approach to estimate the
size of an atom of a non-metallic element is to
measure the distance between two atoms when
they are bound together by a single bond in a
covalent molecule and from this value, the
“Covalent Radius” of the element can be
calculated. For example, the bond distance in
the chlorine molecule (Cl
2
) is 198 pm and half
this distance (99 pm), is taken as the atomic
radius of chlorine. For metals, we define the
term “Metallic Radius” which is taken as half
the internuclear distance separating the metal
cores in the metallic crystal. For example, the
distance between two adjacent copper atoms
in solid copper is 256 pm; hence the metallic
radius of copper is assigned a value of 128 pm.
For simplicity, in this book, we use the term
Atomic Radius to refer to both covalent or
metallic radius depending on whether the
element is a non-metal or a metal. Atomic radii
can be measured by X-ray or other
spectroscopic methods.
The atomic radii of a few elements are listed
in Table 3.6 . Two trends are obvious. We can
explain these trends in terms of nuclear charge
and energy level. The atomic size generally
decreases across a period as illustrated in
Fig. 3.4(a) for the elements of the second period.
It is because within the period the outer
electrons are in the same valence shell and the
effective nuclear charge increases as the atomic
number increases resulting in the increased
attraction of electrons to the nucleus. Within a
family or vertical column of the periodic table,
the atomic radius increases regularly with
atomic number as illustrated in Fig. 3.4(b). For
alkali metals and halogens, as we descend the
groups, the principal quantum number (n)
increases and the valence electrons are farther
from the nucleus. This happens because the
inner energy levels are filled with electrons,
which serve to shield the outer electrons from
the pull of the nucleus. Consequently the size
of the atom increases as reflected in the atomic
radii.
Note that the atomic radii of noble gases
are not considered here. Being monoatomic,
their (non-bonded radii) values are very large.
In fact radii of noble gases should be compared
not with the covalent radii but with the van der
Waals radii of other elements.
Table 3.6(a) Atomic Radii/pm Across the Periods
Table 3.6(b) Atomic Radii/pm Down a Family
Atom Atomic Atom Atomic
(Group I) Radius (Group 17) Radius
Li 152 F 64
Na 186 Cl 99
K 231 Br 114
Rb 244 I 133
Cs 262 At 140
Atom (Period II) Li Be B C N O F
Atomic radius 152 111 88
77 74 66 64
Atom (Period III) Na Mg Al Si P S Cl
Atomic radius 186 160 143 117 110 104 99
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87CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
(b) Ionic Radius
The removal of an electron from an atom results
in the formation of a cation, whereas gain of
an electron leads to an anion. The ionic radii
can be estimated by measuring the distances
between cations and anions in ionic crystals.
In general, the ionic radii of elements exhibit
the same trend as the atomic radii. A cation is
smaller than its parent atom because it has
fewer electrons while its nuclear charge remains
the same. The size of an anion will be larger
than that of the parent atom because the
addition of one or more electrons would result
in increased repulsion among the electrons
and a decrease in effective nuclear charge. For
example, the ionic radius of fluoride ion (F
–
) is
136 pm whereas the atomic radius of fluorine
is only 64 pm. On the other hand, the atomic
radius of sodium is 186 pm compared to the
ionic radius of 95 pm for Na
+
.
When we find some atoms and ions which
contain the same number of electrons, we call
them isoelectronic species*. For example,
O
2–
, F
–
, Na
+
and Mg
2+
have the same number of
electrons (10). Their radii would be different
because of their different nuclear charges. The
cation with the greater positive charge will have
a smaller radius because of the greater
Fig. 3.4 (a) Variation of atomic radius with
atomic number across the second
period
Fig. 3.4 (b) Variation of atomic radius with
atomic number for alkali metals
and halogens
attraction of the electrons to the nucleus. Anion
with the greater negative charge will have the
larger radius. In this case, the net repulsion of
the electrons will outweigh the nuclear charge
and the ion will expand in size.
Problem 3.5
Which of the following species will have
the largest and the smallest size?
Mg, Mg
2+
, Al, Al
3+
.
Solution
Atomic radii decrease across a period.
Cations are smaller than their parent
atoms. Among isoelectronic species, the
one with the larger positive nuclear charge
will have a smaller radius.
Hence the largest species is Mg; the
smallest one is Al
3+
.
(c) Ionization Enthalpy
A quantitative measure of the tendency of an
element to lose electron is given by its
Ionization Enthalpy. It represents the energy
required to remove an electron from an isolated
gaseous atom (X) in its ground state. In other
words, the first ionization enthalpy for an
* Two or more species with same number of atoms, same number of valence electrons and same structure,
regardless of the nature of elements involved.
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88 CHEMISTRY
element X is the enthalpy change (∆
i
H) for the
reaction depicted in equation 3.1.
X(g) → X
+
(g) + e
–
(3.1)
The ionization enthalpy is expressed in
units of kJ mol
–1
. We can define the second
ionization enthalpy as the energy required to
remove the second most loosely bound
electron; it is the energy required to carry out
the reaction shown in equation 3.2.
X
+
(g) → X
2+
(g) + e
–
(3.2)
Energy is always required to remove
electrons from an atom and hence ionization
enthalpies are always positive. The second
ionization enthalpy will be higher than the first
ionization enthalpy because it is more difficult
to remove an electron from a positively charged
ion than from a neutral atom. In the same way
the third ionization enthalpy will be higher than
the second and so on. The term “ionization
enthalpy”, if not qualified, is taken as the first
ionization enthalpy.
The first ionization enthalpies of elements
having atomic numbers up to 60 are plotted
in Fig. 3.5. The periodicity of the graph is quite
striking. You will find maxima at the noble gases
which have closed electron shells and very
stable electron configurations. On the other
hand, minima occur at the alkali metals and
their low ionization enthalpies can be correlated
Fig. 3.5 Variation of first ionization enthalpies
(∆
i
H) with atomic number for elements
with Z = 1 to 60
with their high reactivity. In addition, you will
notice two trends the first ionization enthalpy
generally increases as we go across a period
and decreases as we descend in a group. These
trends are illustrated in Figs. 3.6(a) and 3.6(b)
respectively for the elements of the second
period and the first group of the periodic table.
You will appreciate that the ionization enthalpy
and atomic radius are closely related
properties. To understand these trends, we
have to consider two factors : (i) the attraction
of electrons towards the nucleus, and (ii) the
repulsion of electrons from each other. The
effective nuclear charge experienced by a
valence electron in an atom will be less than
Fig. 3.6(a) First ionization enthalpies (∆
i
H) of elements of the second period as a
function of atomic number (Z) and Fig. 3.6(b) ∆
i
H of alkali metals as a function of Z.
3.6 (a)
3.6 (b)
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89CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
the actual charge on the nucleus because of
“shielding” or “screening” of the valence
electron from the nucleus by the intervening
core electrons. For example, the 2s electron in
lithium is shielded from the nucleus by the
inner core of 1s electrons. As a result, the
valence electron experiences a net positive
charge which is less than the actual charge of
+3. In general, shielding is effective when the
orbitals in the inner shells are completely filled.
This situation occurs in the case of alkali metals
which have single outermost ns-electron
preceded by a noble gas electronic
configuration.
When we move from lithium to fluorine
across the second period, successive electrons
are added to orbitals in the same principal
quantum level and the shielding of the nuclear
charge by the inner core of electrons does not
increase very much to compensate for the
increased attraction of the electron to the
nucleus. Thus, across a period, increasing
nuclear charge outweighs the shielding.
Consequently, the outermost electrons are held
more and more tightly and the ionization
enthalpy increases across a period. As we go
down a group, the outermost electron being
increasingly farther from the nucleus, there is
an increased shielding of the nuclear charge
by the electrons in the inner levels. In this case,
increase in shielding outweighs the increasing
nuclear charge and the removal of the
outermost electron requires less energy down
a group.
From Fig. 3.6(a), you will also notice that
the first ionization enthalpy of boron (Z = 5) is
slightly less than that of beryllium (Z = 4) even
though the former has a greater nuclear charge.
When we consider the same principal quantum
level, an s-electron is attracted to the nucleus
more than a p-electron. In beryllium, the
electron removed during the ionization is an
s-electron whereas the electron removed during
ionization of boron is a p-electron. The
penetration of a 2s-electron to the nucleus is
more than that of a 2
p-electron; hence the 2p
electron of boron is more shielded from the
nucleus by the inner core of electrons than the
2s electrons of beryllium. Therefore, it is easier
to remove the 2p-electron from boron compared
to the removal of a 2s- electron from beryllium.
Thus, boron has a smaller first ionization
enthalpy than beryllium. Another “anomaly”
is the smaller first ionization enthalpy of oxygen
compared to nitrogen. This arises because in
the nitrogen atom, three 2p-electrons reside in
different atomic orbitals (Hund’s rule) whereas
in the oxygen atom, two of the four 2p-electrons
must occupy the same 2p-orbital resulting in
an increased electron-electron repulsion.
Consequently, it is easier to remove the fourth
2p-electron from oxygen than it is, to remove
one of the three 2p-electrons from nitrogen.
Problem 3.6
The first ionization enthalpy (∆
i
H ) values
of the third period elements, Na, Mg and
Si are respectively 496, 737 and 786 kJ
mol
–1
. Predict whether the first ∆
i
H value
for Al will be more close to 575 or 760 kJ
mol
–1
? Justify your answer.
Solution
It will be more close to 575 kJ mol
–1
.
The
value for Al should be lower than that of
Mg because of effective shielding of 3p
electrons from the nucleus by
3s-electrons.
(d) Electron Gain Enthalpy
When an electron is added to a neutral gaseous
atom (X) to convert it into a negative ion, the
enthalpy change accompanying the process is
defined as the Electron Gain Enthalpy (∆
eg
H).
Electron gain enthalpy provides a measure of
the ease with which an atom adds an electron
to form anion as represented by equation 3.3.
X(g) + e
–
→ X
–
(g) (3.3)
Depending on the element, the process of
adding an electron to the atom can be either
endothermic or exothermic. For many elements
energy is released when an electron is added
to the atom and the electron gain enthalpy is
negative. For example, group 17 elements (the
halogens) have very high negative electron gain
enthalpies because they can attain stable noble
gas electronic configurations by picking up an
electron. On the other hand, noble gases have
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90 CHEMISTRY
large positive electron gain enthalpies because
the electron has to enter the next higher
principal quantum level leading to a very
unstable electronic configuration. It may be
noted that electron gain enthalpies have large
negative values toward the upper right of the
periodic table preceding the noble gases.
The variation in electron gain enthalpies of
elements is less systematic than for ionization
enthalpies. As a general rule, electron gain
enthalpy becomes more negative with increase
in the atomic number across a period. The
effective nuclear charge increases from left to
right across a period and consequently it will
be easier to add an electron to a smaller atom
since the added electron on an average would
be closer to the positively charged nucleus. We
should also expect electron gain enthalpy to
become less negative as we go down a group
because the size of the atom increases and the
added electron would be farther from the
nucleus. This is generally the case (Table 3.7).
However, electron gain enthalpy of O or F is
less negative than that of the succeeding
element. This is because when an electron is
added to O or F, the added electron goes to the
smaller n = 2 quantum level and suffers
significant repulsion from the other electrons
present in this level. For the n = 3 quantum
level (S or Cl), the added electron occupies a
larger region of space and the electron-electron
repulsion is much less.
Problem 3.7
Which of the following will have the most
negative electron gain enthalpy and which
the least negative?
P, S, Cl, F.
Explain your answer.
Solution
Electron gain enthalpy generally becomes
more negative across a period as we move
from left to right. Within a group, electron
gain enthalpy becomes less negative down
a group. However, adding an electron to
the 2p-orbital leads to greater repulsion
than adding an electron to the larger
3p-orbital. Hence the element with most
negative electron gain enthalpy is chlorine;
the one with the least negative electron
gain enthalpy is phosphorus.
(e) Electronegativity
A qualitative measure of the ability of an atom
in a chemical compound to attract shared
electrons to itself is called electronegativity.
Unlike ionization enthalpy and electron gain
enthalpy, it is not a measureable quantity.
However, a number of numerical scales of
electronegativity of elements viz., Pauling scale,
Mulliken-Jaffe scale, Allred-Rochow scale have
been developed. The one which is the most
*
In many books, the negative of the enthalpy change for the process depicted in equation 3.3 is defined as the
ELECTRON AFFINITY (A
e
) of the atom under consideration. If energy is released when an electron is added to an atom,
the electron affinity is taken as positive, contrary to thermodynamic convention. If energy has to be supplied to add an
electron to an atom, then the electron affinity of the atom is assigned a negative sign. However, electron affinity is
defined as absolute zero and, therefore at any other temperature (T) heat capacities of the reactants and the products
have to be taken into account in
∆
eg
H = –A
e
– 5/2 RT.
Table 3.7 Electron Gain Enthalpies* / (kJ mol
–1
) of Some Main Group Elements
H – 73 He + 48
Li – 60 O – 141 F – 328 Ne + 116
Na – 53 S – 200 Cl – 349 Ar + 96
K – 48 Se – 195 Br – 325 Kr + 96
Rb – 47 Te – 190 I – 295 Xe + 77
Cs – 46 Po – 174 At – 270 Rn + 68
Group 1
∆
∆
∆
∆
∆
eg
H Group 16
∆
∆
∆
∆
∆
eg
H Group 17
∆
∆
∆∆
∆
eg
H Group 0
∆
∆
∆∆
∆
eg
H
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91CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
widely used is the Pauling scale. Linus Pauling,
an American scientist, in 1922 assigned
arbitrarily a value of 4.0 to fluorine, the element
considered to have the greatest ability to attract
electrons. Approximate values for the
electronegativity of a few elements are given in
Table 3.8(a)
The electronegativity of any given element
is not constant; it varies depending on the
element to which it is bound. Though it is not
a measurable quantity, it does provide a means
electrons and the nucleus increases as the
atomic radius decreases in a period. The
electronegativity also increases. On the same
account electronegativity values decrease with
the increase in atomic radii down a group. The
trend is similar to that of ionization enthalpy.
Knowing the relationship between
electronegativity and atomic radius, can you
now visualise the relationship between
electronegativity and non-metallic properties?
Atom (Period II) Li Be B C N O F
Electronegativity 1.0 1.5
2.0 2.5 3.0 3.5 4.0
Atom (Period III) Na Mg Al Si P S Cl
Electronegativity 0.9 1.2 1.5 1.8 2.1 2.5 3.0
Table 3.8(a) Electronegativity Values (on Pauling scale) Across the Periods
Atom Electronegativity Atom Electronegativity
(Group I) Value (Group 17) Value
Li 1.0 F 4.0
Na 0.9 Cl 3.0
K 0.8 Br 2.8
Rb 0.8 I 2.5
Cs 0.7 At 2.2
Table 3.8(b) Electronegativity Values (on Pauling scale) Down a Family
of predicting the nature of force
that holds a pair of atoms together
– a relationship that you will
explore later.
Electronegativity generally
increases across a period from left
to right (say from lithium to
fluorine) and decrease down a group
(say from fluorine to astatine) in
the periodic table. How can these
trends be explained? Can the
electronegativity be related to
atomic radii, which tend to
decrease across each period from
left to right, but increase down
each group ? The attraction
between the outer (or valence)
Fig. 3.7 The periodic trends of elements in the periodic table
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92 CHEMISTRY
Non-metallic elements have strong tendency
to gain electrons. Therefore, electronegativity
is directly related to that non-metallic
properties of elements. It can be further
extended to say that the electronegativity is
inversely related to the metallic properties of
elements. Thus, the increase in
electronegativities across a period is
accompanied by an increase in non-metallic
properties (or decrease in metallic properties)
of elements. Similarly, the decrease in
electronegativity down a group is accompanied
by a decrease in non-metallic properties (or
increase in metallic properties) of elements.
All these periodic trends are summarised
in figure 3.7.
3.7.2 Periodic Trends in Chemical
Properties
Most of the trends in chemical properties of
elements, such as diagonal relationships, inert
pair effect, effects of lanthanoid contraction etc.
will be dealt with along the discussion of each
group in later units. In this section we shall
study the periodicity of the valence state shown
by elements and the anomalous properties of
the second period elements (from lithium to
fluorine).
(a) Periodicity of Valence or Oxidation
States
The valence is the most characteristic property
of the elements and can be understood in terms
of their electronic configurations. The valence
of representative elements is usually (though
not necessarily) equal to the number of
electrons in the outermost orbitals and / or
equal to eight minus the number of outermost
electrons as shown below.
Nowadays the term oxidation state is
frequently used for valence. Consider the two
oxygen containing compounds: OF
2
and Na
2
O.
The order of electronegativity of the three
elements involved in these compounds is F >
O > Na. Each of the atoms of fluorine, with outer
Group 1 2 13
14 15
16 17 18
Number of valence 1 2 3 4 5 6 7 8
electron
Valence 1 2 3 4 3,5 2,6 1,7 0,8
electronic configuration 2s
2
2p
5
, shares one
electron with oxygen in the OF
2
molecule. Being
highest electronegative element, fluorine is
given oxidation state –1. Since there are two
fluorine atoms in this molecule, oxygen with
outer electronic configuration 2s
2
2p
4
shares
two electrons with fluorine atoms and thereby
exhibits oxidation state +2. In Na
2
O, oxygen
being more electronegative accepts two
electrons, one from each of the two sodium
atoms and, thus, shows oxidation state –2. On
the other hand sodium with electronic
configuration 3s
1
loses one electron to oxygen
and is given oxidation state +1. Thus, the
oxidation state of an element in a particular
compound can be defined as the charge
acquired by its atom on the basis of
electronegative consideration from other atoms
in the molecule.
Problem 3.8
Using the Periodic Table, predict the
formulas of compounds which might be
formed by the following pairs of elements;
(a) silicon and bromine (b) aluminium and
sulphur.
Solution
(a) Silicon is group 14 element with a
valence of 4; bromine belongs to the
halogen family with a valence of 1.
Hence the formula of the compound
formed would be SiBr
4.
(b) Aluminium belongs to group 13 with
a valence of 3; sulphur belongs to
group 16 elements with a valence of
2. Hence, the formula of the compound
formed would be Al
2
S
3.
Some periodic trends observed in the
valence of elements (hydrides and oxides) are
shown in Table 3.9. Other such periodic trends
which occur in the chemical behaviour of the
elements are discussed elsewhere in this book.
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93CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
There are many elements which exhibit variable
valence. This is particularly characteristic of
transition elements and actinoids, which we
shall study later.
(b) Anomalous Properties of Second Period
Elements
The first element of each of the groups 1
(lithium) and 2 (beryllium) and groups 13-17
(boron to fluorine) differs in many respects from
the other members of their respective group.
For example, lithium unlike other alkali metals,
and beryllium unlike other alkaline earth
metals, form compounds with pronounced
covalent character; the other members of these
groups predominantly form ionic compounds.
In fact the behaviour of lithium and beryllium
is more similar with the second element of the
Group 1 2 13 14 15 16 17
Formula LiH B
2
H
6
CH
4
NH
3
H
2
O HF
of hydride NaH CaH
2
AlH
3
SiH
4
PH
3
H
2
S HCl
KH GeH
4
AsH
3
H
2
Se HBr
SnH
4
SbH
3
H
2
Te HI
Formula Li
2
O MgO B
2
O
3
CO
2
N
2
O
3
, N
2
O
5
–
of oxide Na
2
O CaO Al
2
O
3
SiO
2
P
4
O
6
, P
4
O
10
SO
3
Cl
2
O
7
K
2
O SrO Ga
2
O
3
GeO
2
As
2
O
3
, As
2
O
5
SeO
3
–
BaO In
2
O
3
SnO
2
Sb
2
O
3
, Sb
2
O
5
TeO
3
–
PbO
2
Bi
2
O
3
– –
Table 3.9 Periodic Trends in Valence of Elements as shown by the Formulas
of Their Compounds
Property Element
Metallic radius M/ pm Li Be B
152 111 88
Na Mg Al
186 160
143
Li Be
Ionic radius M
+
/ pm 76 31
Na Mg
102 72
following group i.e., magnesium and
aluminium, respectively. This sort of similarity
is commonly referred to as diagonal
relationship in the periodic properties.
What are the reasons for the different
chemical behaviour of the first member of a
group of elements in the s- and p-blocks
compared to that of the subsequent members
in the same group? The anomalous behaviour
is attributed to their small size, large charge/
radius ratio and high electronegativity of the
elements. In addition, the first member of group
has only four valence orbitals (2s and 2p)
available for bonding, whereas the second
member of the groups have nine valence
orbitals (3s, 3p, 3d). As a consequence of this,
the maximum covalency of the first member of
each group is 4 (e.g., boron can only form
BF
4
[ ]
−
, whereas the other members
of the groups can expand their
valence shell to accommodate
more than four pairs of electrons
e.g., aluminium
AlF
6
3
[ ]
−
forms).
Furthermore, the first member of
p-block elements displays greater
ability to form p
π
– p
π
multiple bonds
to itself (e.g., C = C, C ≡ C, N = N,
N ≡ Ν) and to other second period
elements (e.g., C =
O, C = N, C ≡ N,
N = O) compared to subsequent
members of the same group.
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94 CHEMISTRY
Problem 3.9
Are the oxidation state and covalency of
Al in [AlCl(H
2
O)
5
]
2+
same ?
Solution
No. The oxidation state of Al is +3 and the
covalency is 6.
3.7.3 Periodic Trends and Chemical
Reactivity
We have observed the periodic trends in certain
fundamental properties such as atomic and
ionic radii, ionization enthalpy, electron gain
enthalpy and valence. We know by now that
the periodicity is related to electronic
configuration. That is, all chemical and
physical properties are a manifestation of the
electronic configuration of elements. We shall
now try to explore relationships between these
fundamental properties of elements with their
chemical reactivity.
The atomic and ionic radii, as we know,
generally decrease in a period from left to right.
As a consequence, the ionization enthalpies
generally increase (with some exceptions as
outlined in section 3.7.1(a)) and electron gain
enthalpies become more negative across a
period. In other words, the ionization enthalpy
of the extreme left element in a period is the
least and the electron gain enthalpy of the
element on the extreme right is the highest
negative (note : noble gases having completely
filled shells have rather positive electron gain
enthalpy values). This results into high
chemical reactivity at the two extremes and the
lowest in the centre. Thus, the maximum
chemical reactivity at the extreme left (among
alkali metals) is exhibited by the loss of an
electron leading to the formation of a cation
and at the extreme right (among halogens)
shown by the gain of an electron forming an
anion. This property can be related with the
reducing and oxidizing behaviour of the
elements which you will learn later. However,
here it can be directly related to the metallic
and non-metallic character of elements. Thus,
the metallic character of an element, which is
highest at the extremely left decreases and the
non-metallic character increases while moving
from left to right across the period. The
chemical reactivity of an element can be best
shown by its reactions with oxygen and
halogens. Here, we shall consider the reaction
of the elements with oxygen only. Elements on
two extremes of a period easily combine with
oxygen to form oxides. The normal oxide
formed by the element on extreme left is the
most basic (e.g., Na
2
O), whereas that formed
by the element on extreme right is the most
acidic (e.g., Cl
2
O
7
). Oxides of elements in the
centre are amphoteric (e.g., Al
2
O
3
,
As
2
O
3
) or
neutral (e.g., CO, NO, N
2
O). Amphoteric oxides
behave as acidic with bases and as basic with
acids, whereas neutral oxides have no acidic
or basic properties.
Problem 3.10
Show by a chemical reaction with water
that Na
2
O is a basic oxide and Cl
2
O
7
is an
acidic oxide.
Solution
Na
2
O with water forms a strong base
whereas Cl
2
O
7
forms strong acid.
Na
2
O + H
2
O → 2NaOH
Cl
2
O
7
+ H
2
O → 2HClO
4
Their basic or acidic nature can be
qualitatively tested with litmus paper.
Among transition metals (3d series), the change
in atomic radii is much smaller as compared
to those of representative elements across the
period. The change in atomic radii is still
smaller among inner-transition metals
(4f series). The ionization enthalpies are
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95CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
intermediate between those of s- and p-blocks.
As a consequence, they are less electropositive
than group 1 and 2 metals.
In a group, the increase in atomic and ionic
radii with increase in atomic number generally
results in a gradual decrease in ionization
enthalpies and a regular decrease (with
exception in some third period elements as
shown in section 3.7.1(d)) in electron gain
enthalpies in the case of main group elements.
Thus, the metallic character increases down
the group and non-metallic character
decreases. This trend can be related with their
reducing and oxidizing property which you
will learn later. In the case of transition
elements, however, a reverse trend is observed.
This can be explained in terms of atomic size
and ionization enthalpy.
SUMMARY
In this Unit, you have studied the development of the Periodic Law and the Periodic
Table. Mendeleev’s Periodic Table was based on atomic masses. Modern Periodic Table
arranges the elements in the order of their atomic numbers in seven horizontal rows
(periods) and eighteen vertical columns (groups or families). Atomic numbers in a period
are consecutive, whereas in a group they increase in a pattern. Elements of the same
group have similar valence shell electronic configuration and, therefore, exhibit similar
chemical properties. However, the elements of the same period have incrementally
increasing number of electrons from left to right, and, therefore, have different valencies.
Four types of elements can be recognized in the periodic table on the basis of their
electronic configurations. These are s-block, p-block, d-block and f-block elements.
Hydrogen with one electron in the 1s orbital occupies a unique position in the periodic
table. Metals comprise more than seventy eight per cent of the known elements. Non-
metals, which are located at the top of the periodic table, are less than twenty in number.
Elements which lie at the border line between metals and non-metals (e.g., Si, Ge, As)
are called metalloids or semi-metals
. Metallic character increases with increasing atomic
number in a group whereas decreases from left to right in a period. The physical and
chemical properties of elements vary periodically with their atomic numbers.
Periodic trends are observed in atomic sizes, ionization enthalpies, electron
gain enthalpies, electronegativity and valence. The atomic radii decrease while going
from left to right in a period and increase with atomic number in a group. Ionization
enthalpies generally increase across a period and decrease down a group. Electronegativity
also shows a similar trend. Electron gain enthalpies, in general, become more negative
across a period and less negative down a group. There is some periodicity in valence, for
example, among representative elements, the valence is either equal to the number of
electrons in the outermost orbitals or eight minus this number. Chemical reactivity is
highest at the two extremes of a period and is lowest in the centre. The reactivity on the
left extreme of a period is because of the ease of electron loss (or low ionization enthalpy).
Highly reactive elements do not occur in nature in free state; they usually occur in the
combined form. Oxides formed of the elements on the left are basic and of the elements
on the right are acidic in nature. Oxides of elements in the centre are amphoteric or
neutral.
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96 CHEMISTRY
EXERCISES
3.1 What is the basic theme of organisation in the periodic table?
3.2 Which important property did Mendeleev use to classify the elements in his periodic
table and did he stick to that?
3.3 What is the basic difference in approach between the Mendeleev’s Periodic Law
and the Modern Periodic Law?
3.4 On the basis of quantum numbers, justify that the sixth period of the periodic
table should have 32 elements.
3.5 In terms of period and group where would you locate the element with Z =114?
3.6 Write the atomic number of the element present in the third period and seventeenth
group of the periodic table.
3.7 Which element do you think would have been named by
(i) Lawrence Berkeley Laboratory
(ii) Seaborg’s group?
3.8 Why do elements in the same group have similar physical and chemical properties?
3.9 What does atomic radius and ionic radius really mean to you?
3.10 How do atomic radius vary in a period and in a group? How do you explain the
variation?
3.11 What do you understand by isoelectronic species? Name a species that will be
isoelectronic with each of the following atoms or ions.
(i) F
–
(ii) Ar (iii) Mg
2+
(iv) Rb
+
3.12 Consider the following species :
N
3–
, O
2–
, F
–
, Na
+
, Mg
2+
and Al
3+
(a) What is common in them?
(b) Arrange them in the order of increasing ionic radii.
3.13 Explain why cation are smaller and anions larger in radii than their parent atoms?
3.14 What is the significance of the terms — ‘isolated gaseous atom’ and ‘ground state’
while defining the ionization enthalpy and electron gain enthalpy?
Hint : Requirements for comparison purposes.
3.15 Energy of an electron in the ground state of the hydrogen atom is
–2.18×10
–18
J. Calculate the ionization enthalpy of atomic hydrogen in terms of
J mol
–1
.
Hint: Apply the idea of mole concept to derive the answer.
3.16 Among the second period elements the actual ionization enthalpies are in the
order Li < B < Be < C < O < N < F < Ne.
Explain why
(i) Be has higher ∆
i
H than B
(ii) O has lower ∆
i
H than N and F?
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97CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
3.17 How would you explain the fact that the first ionization enthalpy of sodium is
lower than that of magnesium but its second ionization enthalpy is higher than
that of magnesium?
3.18 What are the various factors due to which the ionization enthalpy of the main
group elements tends to decrease down a group?
3.19 The first ionization enthalpy values (in kJ mol
–1
) of group 13 elements are :
B Al Ga In Tl
801 577 579
558 589
How would you explain this deviation from the general trend ?
3.20 Which of the following pairs of elements would have a more negative electron gain
enthalpy?
(i) O or F (ii) F or Cl
3.21 Would you expect the second electron gain enthalpy of O as positive, more negative
or less negative than the first? Justify your answer.
3.22 What is the basic difference between the terms electron gain enthalpy and
electronegativity?
3.23 How would you react to the statement that the electronegativity of N on Pauling
scale is 3.0 in all the nitrogen compounds?
3.24 Describe the theory associated with the radius of an atom as it
(a) gains an electron
(b) loses an electron
3.25 Would you expect the first ionization enthalpies for two isotopes of the same element
to be the same or different? Justify your answer.
3.26 What are the major differences between metals and non-metals?
3.27 Use the periodic table to answer the following questions.
(a) Identify an element with five electrons in the outer subshell.
(b) Identify an element that would tend to lose two electrons.
(c) Identify an element that would tend to gain two electrons.
(d) Identify the group having metal, non-metal, liquid as well as gas at the room
temperature.
3.28 The increasing order of reactivity among group 1 elements is Li < Na < K < Rb <Cs
whereas that among group 17 elements is F > CI > Br > I. Explain.
3.29 Write the general outer electronic configuration of s-, p-, d- and f- block elements.
3.30 Assign the position of the element having outer electronic configuration
(i) ns
2
np
4
for n=3 (ii) (n-1)d
2
ns
2
for n=4, and (iii) (n-2) f
7
(n-1)d
1
ns
2
for n=6, in the
periodic table.
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98 CHEMISTRY
3.31 The first (∆
i
H
1
) and the second (∆
i
H
2
) ionization enthalpies (in kJ mol
–1
)
and the (∆
eg
H) electron gain enthalpy (in kJ mol
–1
) of a few elements are
given below:
Elements ∆H
1
∆H
2
∆
eg
H
I 520 7300 –60
II 419 3051 –48
III 1681 3374 –328
IV 1008 1846 –295
V 2372
5251 +48
VI 738 1451 –40
Which of the above elements is likely to be :
(a) the least reactive element.
(b) the most reactive metal.
(c) the most reactive non-metal.
(d) the least reactive non-metal.
(e) the metal which can form a stable binary halide of the formula
MX
2
(X=halogen).
(f) the metal which can form a predominantly stable covalent halide
of the formula MX (X=halogen)?
3.32 Predict the formulas of the stable binary compounds that would be
formed by the combination of the following pairs of elements.
(a) Lithium and oxygen (b) Magnesium and nitrogen
(c) Aluminium and iodine (d) Silicon and oxygen
(e) Phosphorus and fluorine (f) Element 71 and fluorine
3.33 In the modern periodic table, the period indicates the value of :
(a) atomic number
(b) atomic mass
(c) principal quantum number
(d) azimuthal quantum number.
3.34 Which of the following statements related to the modern periodic table
is incorrect?
(a) The p-block has 6 columns, because a maximum of 6 electrons
can occupy all the orbitals in a p-shell.
(b) The d-block has 8 columns, because a maximum of 8 electrons
can occupy all the orbitals in a d-subshell.
(c) Each block contains a number of columns equal to the number of
electrons that can occupy that subshell.
(d) The block indicates value of azimuthal quantum number (l) for the
last subshell that received electrons in building up the electronic
configuration.
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99CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
3.35 Anything that influences the valence electrons will affect the chemistry of the
element. Which one of the following factors does not affect the valence shell?
(a) Valence principal quantum number (n)
(b) Nuclear charge (Z)
(c) Nuclear mass
(d) Number of core electrons.
3.36 The size of isoelectronic species — F
–
, Ne and Na
+
is affected by
(a) nuclear charge (Z)
(b) valence principal quantum number (n)
(c) electron-electron interaction in the outer orbitals
(d) none of the factors because their size is the same.
3.37 Which one of the following statements is incorrect in relation to ionization
enthalpy?
(a) Ionization enthalpy increases for each successive electron.
(b) The greatest increase in ionization enthalpy is experienced on removal of
electron from core noble gas configuration.
(c) End of valence electrons is marked by a big jump in ionization enthalpy.
(d) Removal of electron from orbitals bearing lower n value is easier than from
orbital having higher n value.
3.38 Considering the elements B, Al, Mg, and K, the correct order of their metallic
character is :
(a) B > Al > Mg > K (b) Al > Mg > B > K
(c) Mg > Al > K > B (d) K > Mg > Al > B
3.39 Considering the elements B, C, N, F, and Si, the correct order of their non-metallic
character is :
(a) B > C > Si > N > F (b) Si > C > B > N > F
(c) F > N > C > B > Si (d) F > N > C > Si > B
3.40 Considering the elements F, Cl, O and N, the correct order of their chemical reactivity
in terms of oxidizing property is :
(a) F > Cl > O > N (b) F > O > Cl > N
(c) Cl > F > O > N (d) O > F > N > Cl
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