299THE s-BLOCK ELEMENTS
The s-block elements of the Periodic Table are those in
which the last electron enters the outermost s-orbital. As
the s-orbital can accommodate only two electrons, two
groups (1 & 2) belong to the s-block of the Periodic Table.
Group 1 of the Periodic Table consists of the elements:
lithium, sodium, potassium, rubidium, caesium and
francium. They are collectively known as the alkali metals.
These are so called because they form hydroxides on
reaction with water which are strongly alkaline in nature.
The elements of Group 2 include beryllium, magnesium,
calcium, strontium, barium and radium. These elements
with the exception of beryllium are commonly known as
the alkaline earth metals. These are so called because their
oxides and hydroxides are alkaline in nature and these
metal oxides are found in the earth’s crust
*.
Among the alkali metals sodium and potassium are
abundant and lithium, rubidium and caesium have much
lower abundances (Table 10.1). Francium is highly
radioactive; its longest-lived isotope
223
Fr has a half-life of
only 21 minutes. Of the alkaline earth metals calcium and
magnesium rank fifth and sixth in abundance respectively
in the earth’s crust. Strontium and barium have much
lower abundances. Beryllium is rare and radium is the
rarest of all comprising only 10
–10
per cent of igneous
rocks
†
(Table 10.2, page 299).
The general electronic configuration of s-block elements
is [noble gas]ns
1
for alkali metals and [noble gas] ns
2
for
alkaline earth metals.
UNIT 10
After studying this unit, you will be
able to
••
••
•
describe the general charact-
eristics of the alkali metals and
their compounds;
••
••
•
explain the general characteristics
of the alkaline earth metals and
their compounds;
••
••
•
describe the manufacture,
properties and uses of industrially
important sodium and calcium
compounds including Portland
cement;
••
••
•
appreciate the biological
significance of sodium,
potassium, magnesium and
calcium.
THE s-BLOCK ELEMENTS
*
The thin, rocky outer layer of the Earth is crust.
†
A type of rock formed
from magma (molten rock) that has cooled and hardened.
The first element of alkali and alkaline earth metals differs
in many respects from the other members of the group
2019-20
300 CHEMISTRY
Lithium and beryllium, the first elements
of Group 1 and Group 2 respectively exhibit
some properties which are different from those
of the other members of the respective group.
In these anomalous properties they resemble
the second element of the following group.
Thus, lithium shows similarities to magnesium
and beryllium to aluminium in many of their
properties. This type of diagonal similarity is
commonly referred to as diagonal relationship
in the periodic table. The diagonal relationship
is due to the similarity in ionic sizes and /or
charge/radius ratio of the elements.
Monovalent sodium and potassium ions and
divalent magnesium and calcium ions are
found in large proportions in biological fluids.
These ions perform important biological
functions such as maintenance of ion balance
and nerve impulse conduction.
10.1 GROUP 1 ELEMENTS: ALKALI
METALS
The alkali metals show regular trends in their
physical and chemical properties with the
increasing atomic number. The atomic,
physical and chemical properties of alkali
metals are discussed below.
10.1.1 Electronic Configuration
All the alkali metals have one valence electron,
ns
1
(Table 10.1) outside the noble gas core.
The loosely held s-electron in the outermost
valence shell of these elements makes them the
most electropositive metals. They readily lose
electron to give monovalent M
+
ions. Hence they
are never found in free state in nature.
increase in atomic number, the atom becomes
larger. The monovalent ions (M
+
) are smaller
than the parent atom. The atomic and ionic
radii of alkali metals increase on moving down
the group i.e., they increase in size while going
from Li to Cs.
10.1.3 Ionization Enthalpy
The ionization enthalpies of the alkali metals
are considerably low and decrease down the
group from Li to Cs. This is because the effect
of increasing size outweighs the increasing
nuclear charge, and the outermost electron is
very well screened from the nuclear charge.
10.1.4 Hydration Enthalpy
The hydration enthalpies of alkali metal ions
decrease with increase in ionic sizes.
Li
+
> Na
+
> K
+
> Rb
+
> Cs
+
Li
+
has maximum degree of hydration and
for this reason lithium salts are mostly
hydrated, e.g., LiCl· 2H
2
O
10.1.5 Physical Properties
All the alkali metals are silvery white, soft and
light metals. Because of the large size, these
elements have low density which increases down
the group from Li to Cs. However, potassium is
lighter than sodium. The melting and boiling
points of the alkali metals are low indicating
weak metallic bonding due to the presence of
only a single valence electron in them. The alkali
metals and their salts impart characteristic
colour to an oxidizing flame. This is because the
heat from the flame excites the outermost orbital
electron to a higher energy level. When the excited
electron comes back to the ground state, there
is emission of radiation in the visible region of
the spectrum as given below:
Alkali metals can therefore, be detected by
the respective flame tests and can be
determined by flame photometry or atomic
absorption spectroscopy. These elements when
irradiated with light, the light energy absorbed
may be sufficient to make an atom lose electron.
Element Symbol Electronic configuration
Lithium Li 1s
2
2s
1
Sodium Na 1s
2
2s
2
2p
6
3s
1
Potassium K 1s
2
2s
2
2p
6
3s
2
3p
6
4s
1
Rubidium Rb 1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
6
5s
1
Caesium 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
Francium Fr [Rn]7s
1
10.1.2 Atomic and Ionic Radii
The alkali metal atoms have the largest sizes
in a particular period of the periodic table. With
Metal Li Na K Rb Cs
Colour Crimson Yellow Violet Red Blue
red violet
λ/nm 670.8 589.2 766.5 780.0 455.5
2019-20
301THE s-BLOCK ELEMENTS
Property Lithium Sodium Potassium Rubidium Caesium Francium
Li Na K Rb Cs Fr
Atomic number 3 11 19 37 55 87
Atomic mass (g mol
–1
) 6.94 22.99 39.10 85.47 132.91 (223)
Electronic [He] 2s
1
[Ne] 3s
1
[Ar] 4s
1
[Kr] 5s
1
[Xe] 6s
1
[Rn] 7s
1
configuration
Ionization 520 496 419 403 376 ~375
enthalpy / kJ mol
–1
Hydration –506 –406 –330 –310 –276 –
enthalpy/kJ mol
–1
Metallic 152 186 227 248 265 –
radius / pm
Ionic radius 76 102 138 152 167 (180)
M
+
/ pm
m.p. / K 454 371 336 312 302 –
b.p / K 1615 1156 1032 961 944 –
Density / g cm
–3
0.53 0.97 0.86 1.53 1.90 –
Standard potentials –3.04 –2.714 –2.925 –2.930 –2.927 –
E
/ V for (M
+
/ M)
Occurrence in 18* 2.27** 1.84** 78-12* 2-6* ~ 10
–18
*
lithosphere
†
This property makes caesium and potassium
useful as electrodes in photoelectric cells.
10.1.6 Chemical Properties
The alkali metals are highly reactive due to
their large size and low ionization enthalpy. The
reactivity of these metals increases down the
group.
(i) Reactivity towards air: The alkali metals
tarnish in dry air due to the formation of
their oxides which in turn react with
moisture to form hydroxides. They burn
vigorously in oxygen forming oxides.
Lithium forms monoxide, sodium forms
peroxide, the other metals form
superoxides. The superoxide O
2
–
ion is
stable only in the presence of large cations
such as K, Rb, Cs.
2 2
4 Li O 2 Li O (oxide)
+→
2 22
2 Na O Na O (peroxide)
+→
2 2
M O MO (superoxide)
+→
(M = K, Rb, Cs)
In all these oxides the oxidation state of the
alkali metal is +1. Lithium shows exceptional
behaviour in reacting directly with nitrogen of
air to form the nitride, Li
3
N as well. Because of
their high reactivity towards air and water,
alkali metals are normally kept in kerosene oil.
Problem 10.1
What is the oxidation state of K in KO
2
?
Solution
The superoxide species is represented as
O
2
–
; since the compound is neutral,
therefore, the oxidation state of potassium
is +1.
*ppm (part per million), ** percentage by weight; † Lithosphere: The Earth’s outer layer: its crust
and part of the upper mantle
Table 10.1 Atomic and Physical Properties of the Alkali Metals
2019-20
302 CHEMISTRY
(ii) Reactivity towards water: The alkali
metals react with water to form hydroxide
and dihydrogen.
2 2
2M 2H O 2M 2OH H
+ −
+ →+ +
(M = an alkali metal)
It may be noted that although lithium has
most negative E
value (Table 10.1), its
reaction with water is less vigorous than
that of sodium which has the least negative
E
value among the alkali metals. This
behaviour of lithium is attributed to its
small size and very high hydration energy.
Other metals of the group react explosively
with water.
They also react with proton donors such
as alcohol, gaseous ammonia and alkynes.
(iii)
Reactivity towards dihydrogen: The
alkali metals react with dihydrogen at
about 673K (lithium at 1073K) to form
hydrides. All the alkali metal hydrides are
ionic solids with high melting points.
2
2M H 2M H
+−
+→
(iv) Reactivity towards halogens : The alkali
metals readily react vigorously with
halogens to form ionic halides, M
+
X
–
.
However, lithium halides are somewhat
covalent. It is because of the high
polarisation capability of lithium ion (The
distortion of electron cloud of the anion by
the cation is called polarisation). The Li
+
ion
is very small in size and has high tendency
to distort electron cloud around the
negative halide ion. Since anion with large
size can be easily distorted, among halides,
lithium iodide is the most covalent in
nature.
(v) Reducing nature: The alkali metals are
strong reducing agents, lithium being the
most and sodium the least powerful
(Table 10.1). The standard electrode
potential (E
) which measures the reducing
power represents the overall change :
2
M(s) M(g) sublimationenthalpy
M(g) M (g) e ionization enthalpy
M (g) H O M (aq) hydration enthalpy
+−
++
→
→+
+→
With the small size of its ion, lithium has
the highest hydration enthalpy which
accounts for its high negative E
value and
its high reducing power.
Problem 10.2
The E
for Cl
2
/Cl
–
is +1.36, for I
2
/I
–
is
+ 0.53, for Ag
+
/Ag is +0.79, Na
+
/Na is
–2.71 and for Li
+
/Li is – 3.04. Arrange
the following ionic species in decreasing
order of reducing strength:
I
–
, Ag, Cl
–
, Li, Na
Solution
The order is Li > Na > I
–
> Ag
> Cl
–
(vi) Solutions in liquid ammonia: The alkali
metals dissolve in liquid ammonia giving
deep blue solutions which are conducting
in nature.
3 3x 3y
M (x y) NH [M(NH ) ] [e(NH ) ]
+ −
++ → +
The blue colour of the solution is due to
the ammoniated electron which absorbs
energy in the visible region of light and thus
imparts blue colour to the solution. The
solutions are paramagnetic and on
standing slowly liberate hydrogen resulting
in the formation of amide.
(am) 3 2(am) 2
M e NH (1) MNH ½H (g)
+ −
++ → +
(where ‘am’ denotes solution in ammonia.)
In concentrated solution, the blue colour
changes to bronze colour and becomes
diamagnetic.
10.1.7 Uses
Lithium metal is used to make useful alloys,
for example with lead to make ‘white metal’
bearings for motor engines, with aluminium
to make aircraft parts, and with magnesium
to make armour plates. It is used in
thermonuclear reactions. Lithium is also used
to make electrochemical cells. Sodium is used
to make a Na/Pb alloy needed to make PbEt
4
and PbMe
4
. These organolead compounds were
earlier used as anti-knock additives to petrol,
but nowadays vehicles use lead-free petrol.
Liquid sodium metal is used as a coolant in
fast breeder nuclear reactors. Potassium has
2019-20
303THE s-BLOCK ELEMENTS
a vital role in biological systems. Potassium
chloride is used as a fertilizer. Potassium
hydroxide is used in the manufacture of soft
soap. It is also used as an excellent absorbent
of carbon dioxide. Caesium is used in devising
photoelectric cells.
10.2 GENERAL CHARACTERISTICS OF
THE COMPOUNDS OF THE ALKALI
METALS
All the common compounds of the alkali metals
are generally ionic in nature. General
characteristics of some of their compounds are
discussed here.
10.2.1 Oxides and Hydroxides
On combustion in excess of air, lithium forms
mainly the oxide, Li
2
O (plus some peroxide
Li
2
O
2
), sodium forms the peroxide, Na
2
O
2
(and
some superoxide NaO
2
) whilst potassium,
rubidium and caesium form the superoxides,
MO
2
. Under appropriate conditions pure
compounds M
2
O, M
2
O
2
and MO
2
may be
prepared. The increasing stability of the
peroxide or superoxide, as the size of the metal
ion increases, is due to the stabilisation of large
anions by larger cations through lattice energy
effects. These oxides are easily hydrolysed by
water to form the hydroxides according to the
following reactions :
–
22
M O H O 2M 2OH
+
+ →+
–
22 2 22
MO 2HO 2M 2OH HO
+
+ →+ +
–
22 22 2
2MO 2H O 2M 2OH H O O
+
+ →+ + +
The oxides and the peroxides are colourless
when pure, but the superoxides are yellow or
orange in colour. The superoxides are also
paramagnetic. Sodium peroxide is widely used
as an oxidising agent in inorganic chemistry.
Problem 10.3
Why is KO
2
paramagnetic ?
Solution
The superoxide O
2
–
is paramagnetic
because of one unpaired electron in π*2p
molecular orbital.
The hydroxides which are obtained by the
reaction of the oxides with water are all white
crystalline solids. The alkali metal hydroxides
are the strongest of all bases and dissolve freely
in water with evolution of much heat on
account of intense hydration.
10.2.2 Halides
The alkali metal halides, MX, (X=F,Cl,Br,I) are
all high melting, colourless crystalline solids.
They can be prepared by the reaction of the
appropriate oxide, hydroxide or carbonate with
aqueous hydrohalic acid (HX). All of these
halides have high negative enthalpies of
formation; the ∆
f
H
values for fluorides
become less negative as we go down the group,
whilst the reverse is true for ∆
f
H
for chlorides,
bromides and iodides. For a given metal
∆
f
H
always becomes less negative from
fluoride to iodide.
The melting and boiling points always
follow the trend: fluoride > chloride > bromide
> iodide. All these halides are soluble in water.
The low solubility of LiF in water is due to its
high lattice enthalpy whereas the low solubility
of CsI is due to smaller hydration enthalpy of
its two ions. Other halides of lithium are soluble
in ethanol, acetone and ethylacetate; LiCl is
soluble in pyridine also.
10.2.3 Salts of Oxo-Acids
Oxo-acids are those in which the acidic proton
is on a hydroxyl group with an oxo group
attached to the same atom e.g., carbonic acid,
H
2
CO
3
(OC(OH)
2
; sulphuric acid, H
2
SO
4
(O
2
S(OH)
2
). The alkali metals form salts with
all the oxo-acids. They are generally soluble
in water and thermally stable. Their
carbonates (M
2
CO
3
) and in most cases the
hydrogencarbonates (MHCO
3
) also are highly
stable to heat. As the electropositive character
increases down the group, the stability of the
carbonates and hydorgencarbonates increases.
Lithium carbonate is not so stable to heat;
lithium being very small in size polarises a
large CO
3
2–
ion leading to the formation of more
stable Li
2
O and CO
2
. Its hydrogencarbonate
does not exist as a solid.
2019-20
304 CHEMISTRY
10.3 ANOMALOUS PROPERTIES OF
LITHIUM
The anomalous behaviour of lithium is due to
the : (i) exceptionally small size of its atom and
ion, and (ii) high polarising power (i.e., charge/
radius ratio). As a result, there is increased
covalent character of lithium compounds which
is responsible for their solubility in organic
solvents. Further, lithium shows diagonal
relationship to magnesium which has been
discussed subsequently.
10.3.1 Points of Difference between
Lithium and other Alkali Metals
(i) Lithium is much harder. Its m.p. and b.p.
are higher than the other alkali metals.
(ii) Lithium is least reactive but the strongest
reducing agent among all the alkali metals.
On combustion in air it forms mainly
monoxide, Li
2
O and the nitride, Li
3
N unlike
other alkali metals.
(iii) LiCl is deliquescent and crystallises as a
hydrate, LiCl.2H
2
O whereas other alkali
metal chlorides do not form hydrates.
(iv) Lithium hydrogencarbonate is not
obtained in the solid form while all other
elements form solid hydrogencarbonates.
(v) Lithium unlike other alkali metals forms
no ethynide on reaction with ethyne.
(vi) Lithium nitrate when heated gives lithium
oxide, Li
2
O, whereas other alkali metal
nitrates decompose to give the
corresponding nitrite.
3 2 22
4LiNO 2 Li O 4 NO O
→++
2
3 22
NaNO 2 NaNO O
→+
(vii) LiF and Li
2
O are comparatively much less
soluble in water than the corresponding
compounds of other alkali metals.
10.3.2 Points of Similarities between
Lithium and Magnesium
The similarity between lithium and magnesium
is particularly striking and arises because of
their similar sizes : atomic radii, Li = 152 pm,
Mg = 160 pm; ionic radii : Li
+
= 76 pm,
Mg
2+
= 72 pm. The main points of similarity are:
(i) Both lithium and magnesium are harder
and lighter than other elements in the
respective groups.
(ii) Lithium and magnesium react slowly with
water. Their oxides and hydroxides are
much less soluble and their hydroxides
decompose on heating. Both form a nitride,
Li
3
N and Mg
3
N
2
, by direct combination
with nitrogen.
(iii) The oxides, Li
2
O and MgO do not combine
with excess oxygen to give any superoxide.
(iv) The carbonates of lithium and magnesium
decompose easily on heating to
form the oxides and CO
2
. Solid
hydrogencarbonates are not formed by
lithium and magnesium.
(v) Both LiCl and MgCl
2
are soluble in ethanol.
(vi) Both LiCl and MgCl
2
are deliquescent and
crystallise from aqueous solution as
hydrates, LiCl·2H
2
O and MgCl
2
·8H
2
O.
10.4 SOME IMPORTANT COMPOUNDS OF
SODIUM
Industrially important compounds of sodium
include sodium carbonate, sodium hydroxide,
sodium chloride and sodium bicarbonate. The
large scale production of these compounds
and their uses are described below:
Sodium Carbonate (Washing Soda),
Na
2
CO
3
·10H
2
O
Sodium carbonate is generally prepared by
Solvay Process. In this process, advantage is
taken of the low solubility of sodium
hydrogencarbonate whereby it gets
precipitated in the reaction of sodium chloride
with ammonium hydrogencarbonate. The
latter is prepared by passing CO
2
to a
concentrated solution of sodium chloride
saturated with ammonia, where ammonium
carbonate followed by ammonium
hydrogencarbonate are formed. The equations
for the complete process may be written as :
(
)
32 2 43
2
2 NH H O CO NH CO
++→
(
)
4 32 2 43
2
NH CO H O CO 2 NH HCO
++→
43
4 3
NH HCO NaCl NH Cl NaHCO
+→ +
Sodium hydrogencarbonate crystal
separates. These are heated to give sodium
carbonate.
2019-20
305THE s-BLOCK ELEMENTS
3 23 2 2
2 NaHCO Na CO CO H O
→ ++
In this process NH
3
is recovered when the
solution containing NH
4
Cl is treated with
Ca(OH)
2
. Calcium chloride is obtained as a
by-product.
(
)
4 3 22
2
2 NH Cl Ca OH 2 NH CaCl H O
+ →++
It may be mentioned here that Solvay
process cannot be extended to the
manufacture of potassium carbonate because
potassium hydrogencarbonate is too soluble
to be precipitated by the addition of
ammonium hydrogencarbonate to a saturated
solution of potassium chloride.
Properties : Sodium carbonate is a white
crystalline solid which exists as a decahydrate,
Na
2
CO
3
·10H
2
O. This is also called washing
soda. It is readily soluble in water. On heating,
the decahydrate loses its water of crystallisation
to form monohydrate. Above 373K, the
monohydrate becomes completely anhydrous
and changes to a white powder called soda ash.
375 K
23 2 2 32 2
Na CO 10H O Na CO H O 9H O
→ +
373K
232 23 2
Na CO H O Na CO H O
>
→ +
Carbonate part of sodium carbonate gets
hydrolysed by water to form an alkaline
solution.
2– ––
32 3
CO H O HCO OH
+→ +
Uses:
(i) It is used in water softening, laundering
and cleaning.
(ii) It is used in the manufacture of glass,
soap, borax and caustic soda.
(iii) It is used in paper, paints and textile
industries.
(iv) It is an important laboratory reagent both
in qualitative and quantitative analysis.
Sodium Chloride, NaCl
The most abundant source of sodium chloride
is sea water which contains 2.7 to 2.9% by
mass of the salt. In tropical countries like India,
common salt is generally obtained by
evaporation of sea water. Approximately 50
lakh tons of salt are produced annually in
India by solar evaporation. Crude sodium
chloride, generally obtained by crystallisation
of brine solution, contains sodium sulphate,
calcium sulphate, calcium chloride and
magnesium chloride as impurities. Calcium
chloride, CaCl
2
, and magnesium chloride,
MgCl
2
are impurities because they are
deliquescent (absorb moisture easily from the
atmosphere). To obtain pure sodium chloride,
the crude salt is dissolved in minimum amount
of water and filtered to remove insoluble
impurities. The solution is then saturated with
hydrogen chloride gas. Crystals of pure
sodium chloride separate out. Calcium and
magnesium chloride, being more soluble than
sodium chloride, remain in solution.
Sodium chloride melts at 1081K. It has a
solubility of 36.0 g in 100 g of water at 273 K.
The solubility does not increase appreciably
with increase in temperature.
Uses :
(i) It is used as a common salt or table salt for
domestic purpose.
(ii) It is used for the preparation of Na
2
O
2
,
NaOH and Na
2
CO
3
.
Sodium Hydroxide (Caustic Soda), NaOH
Sodium hydroxide is generally prepared
commercially by the electrolysis of sodium
chloride in Castner-Kellner cell. A brine
solution is electrolysed using a mercury
cathode and a carbon anode. Sodium metal
discharged at the cathode combines with
mercury to form sodium amalgam. Chlorine
gas is evolved at the anode.
Hg
Cathode : Na e Na – amalgam
+−
+ →
– –
2
1
Anode : Cl Cl e
2
→+
The amalgam is treated with water to give
sodium hydroxide and hydrogen gas.
2Na-amalgam + 2H
2
O2NaOH+ 2Hg +H
2
Sodium hydroxide is a white, translucent
solid. It melts at 591 K. It is readily soluble in
water to give a strong alkaline solution.
Crystals of sodium hydroxide are deliquescent.
The sodium hydroxide solution at the surface
reacts with the CO
2
in the atmosphere to form
Na
2
CO
3
.
2019-20
306 CHEMISTRY
Uses: It is used in (i) the manufacture of soap,
paper, artificial silk and a number of chemicals,
(ii) in petroleum refining, (iii) in the purification
of bauxite, (iv) in the textile industries for
mercerising cotton fabrics, (v) for the
preparation of pure fats and oils, and (vi) as a
laboratory reagent.
Sodium Hydrogencarbonate (Baking
Soda), NaHCO
3
Sodium hydrogencarbonate is known as
baking soda because it decomposes on heating
to generate bubbles of carbon dioxide (leaving
holes in cakes or pastries and making them
light and fluffy).
Sodium hydrogencarbonate is made by
saturating a solution of sodium carbonate with
carbon dioxide. The white crystalline powder
of sodium hydrogencarbonate, being less
soluble, gets separated out.
23 2 2 3
Na CO H O CO 2 NaHCO
++→
Sodium hydrogencarbonate is a mild
antiseptic for skin infections. It is used in fire
extinguishers.
10.5 BIOLOGICAL IMPORTANCE OF
SODIUM AND POTASSIUM
A typical 70 kg man contains about 90 g of Na
and 170 g of K compared with only 5 g of iron
and 0.06 g of copper.
Sodium ions are found primarily on the
outside of cells, being located in blood plasma
and in the interstitial fluid which surrounds
the cells. These ions participate in the
transmission of nerve signals, in regulating the
flow of water across cell membranes and in the
transport of sugars and amino acids into cells.
Sodium and potassium, although so similar
chemically, differ quantitatively in their ability
to penetrate cell membranes, in their transport
mechanisms and in their efficiency to activate
enzymes. Thus, potassium ions are the most
abundant cations within cell fluids, where they
activate many enzymes, participate in the
oxidation of glucose to produce ATP and, with
sodium, are responsible for the transmission
of nerve signals.
There is a very considerable variation in the
concentration of sodium and potassium ions
found on the opposite sides of cell membranes.
As a typical example, in blood plasma, sodium
is present to the extent of 143 mmolL
–1
,
whereas the potassium level is only
5 mmolL
–1
within the red blood cells. These
concentrations change to 10 mmolL
–1
(Na
+
) and
105 mmolL
–1
(K
+
). These ionic gradients
demonstrate that a discriminatory mechanism,
called the sodium-potassium pump, operates
across the cell membranes which consumes
more than one-third of the ATP used by a
resting animal and about 15 kg per 24 h in a
resting human.
10.6 GROUP 2 ELEMENTS : ALKALINE
EARTH METALS
The group 2 elements comprise beryllium,
magnesium, calcium, strontium, barium and
radium. They follow alkali metals in the
periodic table. These (except beryllium) are
known as alkaline earth metals. The first
element beryllium differs from the rest of the
members and shows diagonal relationship to
aluminium. The atomic and physical
properties of the alkaline earth metals are
shown in Table 10.2.
10.6.1 Electronic Configuration
These elements have two electrons in the
s -orbital of the valence shell (Table 10.2). Their
general electronic configuration may be
represented as [noble gas] ns
2
. Like alkali
metals, the compounds of these elements are
also predominantly ionic.
Element Symbol Electronic
configuration
Beryllium Be 1s
2
2s
2
Magnesium Mg 1s
2
2s
2
2p
6
3s
2
Calcium Ca 1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
Strontium Sr 1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
6
5s
2
Barium Ba 1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
6
4d
10
5s
2
5p
6
6s
2
or
[Xe]6s
2
Radium Ra [Rn]7s
2
10.6.2 Atomic and Ionic Radii
The atomic and ionic radii of the alkaline earth
metals are smaller than those of the
2019-20
307THE s-BLOCK ELEMENTS
corresponding alkali metals in the same
periods. This is due to the increased nuclear
charge in these elements. Within the group, the
atomic and ionic radii increase with increase
in atomic number.
10.6.3 Ionization Enthalpies
The alkaline earth metals have low ionization
enthalpies due to fairly large size of the atoms.
Since the atomic size increases down the
group, their ionization enthalpy decreases
(Table 10.2). The first ionisation enthalpies of
the alkaline earth metals are higher than those
of the corresponding Group 1 metals. This is
due to their small size as compared to the
corresponding alkali metals. It is interesting
to note that the second ionisation enthalpies
of the alkaline earth metals are smaller than
those of the corresponding alkali metals.
10.6.4 Hydration Enthalpies
Like alkali metal ions, the hydration enthalpies
of alkaline earth metal ions decrease with
increase in ionic size down the group.
Be
2+
> Mg
2+
> Ca
2+
> Sr
2+
> Ba
2+
The hydration enthalpies of alkaline earth
metal ions are larger than those of alkali metal
ions. Thus, compounds of alkaline earth metals
are more extensively hydrated than those of
alkali metals, e.g., MgCl
2
and CaCl
2
exist as
MgCl
2
.6H
2
O and CaCl
2
· 6H
2
O while NaCl and
KCl do not form such hydrates.
10.6.5 Physical Properties
The alkaline earth metals, in general, are silvery
white, lustrous and relatively soft but harder
than the alkali metals. Beryllium and
magnesium appear to be somewhat greyish.
The melting and boiling points of these metals
are higher than the corresponding alkali metals
due to smaller sizes. The trend is, however, not
systematic. Because of the low ionisation
enthalpies, they are strongly electropositive in
nature. The electropositive character increases
down the group from Be to Ba. Calcium,
Property Beryllium Magnesium Calcium Strontium Barium Radium
Be Mg Ca Sr Ba Ra
Atomic number 4 12 20 38 56 88
Atomic mass (g mol
–1
) 9.01 24.31 40.08 87.62 137.33 226.03
Electronic [He] 2s
2
[Ne] 3s
2
[Ar] 4s
2
[Kr] 5s
2
[Xe] 6s
2
[Rn] 7s
2
configuration
Ionization 899 737 590 549 503 509
enthalpy (I) / kJ mol
–1
Ionization 1757 1450 1145 1064 965 979
enthalpy (II) /kJ mol
–1
Hydration enthalpy – 2494 – 1921 –1577 – 1443 – 1305 –
(kJ/mol)
Metallic 111 160 197 215 222 –
radius / pm
Ionic radius 31 72 100 118 135 148
M
2+
/ pm
m.p. / K 1560 924 1124 1062 1002 973
b.p / K 2745 1363 1767 1655 2078 (1973)
Density / g cm
–3
1.84 1.74 1.55 2.63 3.59 (5.5)
Standard potential –1.97 –2.36 –2.84 –2.89 – 2.92 –2.92
E
/ V for (M
2+
/ M)
Occurrence in 2* 2.76** 4.6** 384* 390 * 10
–6
*
lithosphere
Table 10.2 Atomic and Physical Properties of the Alkaline Earth Metals
*ppm (part per million); ** percentage by weight
2019-20
308 CHEMISTRY
strontium and barium impart characteristic
brick red, crimson and apple green colours
respectively to the flame. In flame the electrons
are excited to higher energy levels and when
they drop back to the ground state, energy is
emitted in the form of visible light. The
electrons in beryllium and magnesium are too
strongly bound to get excited by flame. Hence,
these elements do not impart any colour to the
flame. The flame test for Ca, Sr and Ba is
helpful in their detection in qualitative analysis
and estimation by flame photometry. The
alkaline earth metals like those of alkali metals
have high electrical and thermal conductivities
which are typical characteristics of metals.
10.6.6 Chemical Properties
The alkaline earth metals are less reactive than
the alkali metals. The reactivity of these
elements increases on going down the group.
(i) Reactivity towards air and water:
Beryllium and magnesium are kinetically inert
to oxygen and water because of the formation
of an oxide film on their surface. However,
powdered beryllium burns brilliantly on
ignition in air to give BeO and Be
3
N
2
.
Magnesium is more electropositive and burns
with dazzling brilliance in air to give MgO and
Mg
3
N
2
. Calcium, strontium and barium are
readily attacked by air to form the oxide and
nitride. They also react with water with
increasing vigour even in cold to form
hydroxides.
(ii) Reactivity towards the halogens: All
the alkaline earth metals combine with halogen
at elevated temperatures forming their halides.
(
)
2 2
M X MX X F, Cl, Br, l
+→ =
Thermal decomposition of (NH
4
)
2
BeF
4
is the
best route for the preparation of BeF
2
, and
BeCl
2
is conveniently made from the oxide.
600 800 K
22
BeO C Cl BeCl CO
−
++ +
(iii) Reactivity towards hydrogen: All the
elements except beryllium combine with
hydrogen upon heating to form their hydrides,
MH
2.
BeH
2
, however, can be prepared by the reaction
of BeCl
2
with LiAlH
4
.
2 4 2 3
2BeCl LiAlH 2BeH LiCl AlCl
+ → ++
(iv) Reactivity towards acids: The alkaline
earth metals readily react with acids liberating
dihydrogen.
M + 2HCl → MCl
2
+ H
2
(v) Reducing nature: Like alkali metals, the
alkaline earth metals are strong reducing
agents. This is indicated by large negative
values of their reduction potentials
(Table 10.2). However their reducing power is
less than those of their corresponding alkali
metals. Beryllium has less negative value
compared to other alkaline earth metals.
However, its reducing nature is due to large
hydration energy associated with the small
size of Be
2+
ion and relatively large value of the
atomization enthalpy of the metal.
(vi) Solutions in liquid ammonia: Like
alkali metals, the alkaline earth metals dissolve
in liquid ammonia to give deep blue black
solutions forming ammoniated ions.
()
() ()
2 –
3 3 3
X Y
M x y NH M NH 2 e NH
+
++ → +
From these solutions, the ammoniates,
[M(NH
3
)
6
]
2+
can be recovered.
10.6.7 Uses
Beryllium is used in the manufacture of alloys.
Copper-beryllium alloys are used in the
preparation of high strength springs. Metallic
beryllium is used for making windows of
X-ray tubes. Magnesium forms alloys with
aluminium, zinc, manganese and tin.
Magnesium-aluminium alloys being light in
mass are used in air-craft construction.
Magnesium (powder and ribbon) is used in
flash powders and bulbs, incendiary bombs
and signals. A suspension of magnesium
hydroxide in water (called milk of magnesia)
is used as antacid in medicine. Magnesium
carbonate is an ingredient of toothpaste.
Calcium is used in the extraction of metals from
oxides which are difficult to reduce with
carbon. Calcium and barium metals, owing
to their reactivity with oxygen and nitrogen at
elevated temperatures, have often been used
to remove air from vacuum tubes. Radium
salts are used in radiotherapy, for example, in
the treatment of cancer.
2019-20
309THE s-BLOCK ELEMENTS
10.7 GENERAL CHARACTERISTICS OF
COMPOUNDS OF THE ALKALINE
EARTH METALS
The dipositive oxidation state (M
2+
) is the
predominant valence of Group 2 elements. The
alkaline earth metals form compounds which
are predominantly ionic but less ionic than the
corresponding compounds of alkali metals.
This is due to increased nuclear charge and
smaller size. The oxides and other compounds
of beryllium and magnesium are more covalent
than those formed by the heavier and large
sized members (Ca, Sr, Ba). The general
characteristics of some of the compounds of
alkali earth metals are described below.
(i) Oxides and Hydroxides: The alkaline
earth metals burn in oxygen to form the
monoxide, MO which, except for BeO, have
rock-salt structure. The BeO is essentially
covalent in nature. The enthalpies of formation
of these oxides are quite high and consequently
they are very stable to heat. BeO is amphoteric
while oxides of other elements are ionic in
nature. All these oxides except BeO are basic
in nature and react with water to form sparingly
soluble hydroxides.
MO + H
2
O → M(OH)
2
The solubility, thermal stability and the
basic character of these hydroxides increase
with increasing atomic number from Mg(OH)
2
to Ba(OH)
2
. The alkaline earth metal
hydroxides are, however, less basic and less
stable than alkali metal hydroxides. Beryllium
hydroxide is amphoteric in nature as it reacts
with acid and alkali both.
Be(OH)
2
+ 2OH
–
→ [Be(OH)
4
]
2–
Beryllate ion
Be(OH)
2
+ 2HCl + 2H
2
O → [Be(OH)
4
]Cl
2
(ii) Halides: Except for beryllium halides, all
other halides of alkaline earth metals are ionic
in nature. Beryllium halides are essentially
covalent and soluble in organic solvents.
Beryllium chloride has a chain structure in the
solid state as shown below:
In the vapour phase BeCl
2
tends to form a
chloro-bridged dimer which dissociates into the
linear monomer at high temperatures of the
order of 1200 K. The tendency to form halide
hydrates gradually decreases (for example,
MgCl
2
·8H
2
O, CaCl
2
·6H
2
O, SrCl
2
·6H
2
O and
BaCl
2
·2H
2
O) down the group. The dehydration
of hydrated chlorides, bromides and iodides
of Ca, Sr and Ba can be achieved on heating;
however, the corresponding hydrated halides
of Be and Mg on heating suffer hydrolysis. The
fluorides are relatively less soluble than the
chlorides owing to their high lattice energies.
(iii) Salts of Oxoacids: The alkaline earth
metals also form salts of oxoacids. Some of
these are :
Carbonates: Carbonates of alkaline earth
metals are insoluble in water and can be
precipitated by addition of a sodium or
ammonium carbonate solution to a solution
of a soluble salt of these metals. The solubility
of carbonates in water decreases as the atomic
number of the metal ion increases. All the
carbonates decompose on heating to give
carbon dioxide and the oxide. Beryllium
carbonate is unstable and can be kept only in
the atmosphere of CO
2
. The thermal stability
increases with increasing cationic size.
Sulphates: The sulphates of the alkaline earth
metals are all white solids and stable to heat.
BeSO
4
, and MgSO
4
are readily soluble in water;
the solubility decreases from CaSO
4
to BaSO
4
.
The greater hydration enthalpies of Be
2+
and
Mg
2+
ions overcome the lattice enthalpy factor
and therefore their sulphates are soluble in
water.
Nitrates: The nitrates are made by dissolution
of the carbonates in dilute nitric acid.
Magnesium nitrate crystallises with six
molecules of water, whereas barium nitrate
crystallises as the anhydrous salt. This again
shows a decreasing tendency to form hydrates
with increasing size and decreasing hydration
enthalpy. All of them decompose on heating to
give the oxide like lithium nitrate.
(
)
3 22
2
2M NO 2MO 4NO O
→+ +
(M = Be, Mg, Ca, Sr, Ba)
2019-20
310 CHEMISTRY
Problem 10.4
Why does the solubility of alkaline earth
metal hydroxides in water increase down
the group?
Solution
Among alkaline earth metal hydroxides,
the anion being common the cationic
radius will influence the lattice enthalpy.
Since lattice enthalpy decreases much
more than the hydration enthalpy with
increasing ionic size, the solubility
increases as we go down the group.
Problem 10.5
Why does the solubility of alkaline earth
metal carbonates and sulphates in water
decrease down the group?
Solution
The size of anions being much larger
compared to cations, the lattice enthalpy
will remain almost constant within a
particular group. Since the hydration
enthalpies decrease down the group,
solubility will decrease as found for
alkaline earth metal carbonates and
sulphates.
10.8 ANOMALOUS BEHAVIOUR OF
BERYLLIUM
Beryllium, the first member of the Group 2
metals, shows anomalous behaviour as
compared to magnesium and rest of the
members. Further, it shows diagonal
relationship to aluminium which is discussed
subsequently.
(i) Beryllium has exceptionally small atomic
and ionic sizes and thus does not compare
well with other members of the group.
Because of high ionisation enthalpy and
small size it forms compounds which are
largely covalent and get easily hydrolysed.
(ii) Beryllium does not exhibit coordination
number more than four as in its valence
shell there are only four orbitals. The
remaining members of the group can have
a coordination number of six by making
use of d-orbitals.
(iii) The oxide and hydroxide of beryllium,
unlike the hydroxides of other elements in
the group, are amphoteric in nature.
10.8.1 Diagonal Relationship between
Beryllium and Aluminium
The ionic radius of Be
2+
is estimated to be
31 pm; the charge/radius ratio is nearly the
same as that of the Al
3+
ion. Hence beryllium
resembles aluminium in some ways. Some of
the similarities are:
(i) Like aluminium, beryllium is not readily
attacked by acids because of the presence
of an oxide film on the surface of the metal.
(ii) Beryllium hydroxide dissolves in excess of
alkali to give a beryllate ion, [Be(OH)
4
]
2–
just
as aluminium hydroxide gives aluminate
ion, [Al(OH)
4
]
–
.
(iii) The chlorides of both beryllium and
aluminium have Cl
–
bridged chloride
structure in vapour phase. Both the
chlorides are soluble in organic solvents
and are strong Lewis acids. They are used
as Friedel Craft catalysts.
(iv) Beryllium and aluminium ions have strong
tendency to form complexes, BeF
4
2–
, AlF
6
3–
.
10.9 SOME IMPORTANT COMPOUNDS OF
CALCIUM
Important compounds of calcium are calcium
oxide, calcium hydroxide, calcium sulphate,
calcium carbonate and cement. These are
industrially important compounds. The large
scale preparation of these compounds and
their uses are described below.
Calcium Oxide or Quick Lime, CaO
It is prepared on a commercial scale by
heating limestone (CaCO
3
) in a rotary kiln at
1070-1270 K.
heat
3 2
CaCO CaO CO
+
The carbon dioxide is removed as soon as
it is produced to enable the reaction to proceed
to completion.
Calcium oxide is a white amorphous solid.
It has a melting point of 2870 K. On exposure
to atmosphere, it absorbs moisture and carbon
dioxide.
2019-20
311THE s-BLOCK ELEMENTS
(
)
2
2
CaO H O Ca OH
+→
2 3
CaO CO CaCO
+→
The addition of limited amount of water
breaks the lump of lime. This process is called
slaking of lime. Quick lime slaked with soda
gives solid sodalime. Being a basic oxide, it
combines with acidic oxides at high
temperature.
2 3
CaO SiO CaSiO
+→
(
)
4 10
34
2
6CaO P O 2Ca PO+→
Uses:
(i) It is an important primary material for
manufacturing cement and is the cheapest
form of alkali.
(ii) It is used in the manufacture of sodium
carbonate from caustic soda.
(iii) It is employed in the purification of sugar
and in the manufacture of dye stuffs.
Calcium Hydroxide (Slaked lime), Ca(OH)
2
Calcium hydroxide is prepared by adding
water to quick lime, CaO.
It is a white amorphous powder. It is
sparingly soluble in water. The aqueous
solution is known as lime water and a
suspension of slaked lime in water is known
as milk of lime.
When carbon dioxide is passed through
lime water it turns milky due to the formation
of calcium carbonate.
(
)
2
32
2
Ca OH CO CaCO H O
+→ +
On passing excess of carbon dioxide, the
precipitate dissolves to form calcium
hydrogencarbonate.
(
)
3 22 3
2
CaCO CO H O Ca HCO++→
Milk of lime reacts with chlorine to form
hypochlorite, a constituent of bleaching
powder.
()
(
)
2 2
2
2
2
Bleaching powder
2Ca OH 2Cl 2H O
CaCl Ca OCl
+→ +
+
Uses:
(i) It is used in the preparation of mortar, a
building material.
(ii) It is used in white wash due to its
disinfectant nature.
(iii) It is used in glass making, in tanning
industry, for the preparation of bleaching
powder and for purification of sugar.
Calcium Carbonate, CaCO
3
Calcium carbonate occurs in nature in several
forms like limestone, chalk, marble etc. It can
be prepared by passing carbon dioxide
through slaked lime or by the addition of
sodium carbonate to calcium chloride.
(
)
2
32
2
Ca OH CO CaCO H O
+→ +
2 23 3
CaCl Na CO CaCO 2NaCl
+ →+
Excess of carbon dioxide should be
avoided since this leads to the formation of
water soluble calcium hydrogencarbonate.
Calcium carbonate is a white fluffy powder.
It is almost insoluble in water. When heated
to 1200 K, it decomposes to evolve carbon
dioxide.
1200K
3
2
CaCO CaO CO
→ +
It reacts with dilute acid to liberate carbon
dioxide.
3 22 2
CaCO 2HCl CaCl H O CO
+ → ++
3 24 42 2
CaCO H SO CaSO H O CO
+ → ++
Uses:
It is used as a building material in the form of
marble and in the manufacture of quick lime.
Calcium carbonate along with magnesium
carbonate is used as a flux in the extraction of
metals such as iron. Specially precipitated
CaCO
3
is extensively used in the manufacture
of high quality paper. It is also used as an
antacid, mild abrasive in tooth paste, a
constituent of chewing gum, and a filler in
cosmetics.
Calcium Sulphate (Plaster of Paris),
CaSO
4
·½ H
2
O
It is a hemihydrate of calcium sulphate. It is
obtained when gypsum, CaSO
4
·2H
2
O, is
heated to 393 K.
(
)
(
)
42 42 2
2 CaSO .2H O 2 CaSO .H O 3H O→ +
Above 393 K, no water of crystallisation is left
and anhydrous calcium sulphate, CaSO
4
is
formed. This is known as ‘dead burnt plaster’.
2019-20
312 CHEMISTRY
It has a remarkable property of setting with
water. On mixing with an adequate quantity
of water it forms a plastic mass that gets into a
hard solid in 5 to 15 minutes.
Uses:
The largest use of Plaster of Paris is in the
building industry as well as plasters. It is used
for immoblising the affected part of organ where
there is a bone fracture or sprain. It is also
employed in dentistry, in ornamental work and
for making casts of statues and busts.
Cement: Cement is an important building
material. It was first introduced in England in
1824 by Joseph Aspdin. It is also called
Portland cement because it resembles with the
natural limestone quarried in the Isle of
Portland, England.
Cement is a product obtained by
combining a material rich in lime, CaO with
other material such as clay which contains
silica, SiO
2
along with the oxides of
aluminium, iron and magnesium. The average
composition of Portland cement is : CaO, 50-
60%; SiO
2
, 20-25%; Al
2
O
3
, 5-10%; MgO, 2-
3%; Fe
2
O
3
, 1-2% and SO
3
, 1-2%. For a good
quality cement, the ratio of silica (SiO
2
) to
alumina (Al
2
O
3
) should be between 2.5 and 4
and the ratio of lime (CaO) to the total of the
oxides of silicon (SiO
2
) aluminium (Al
2
O
3
)
and iron (Fe
2
O
3
) should be as close as possible
to 2.
The raw materials for the manufacture of
cement are limestone and clay. When clay and
lime are strongly heated together they fuse and
react to form ‘cement clinker’. This clinker is
mixed with 2-3% by weight of gypsum
(CaSO
4
·2H
2
O) to form cement. Thus important
ingredients present in Portland cement are
dicalcium silicate (Ca
2
SiO
4
) 26%, tricalcium
SUMMARY
The s-Block of the periodic table constitutes Group1 (alkali metals) and Group 2
(alkaline earth metals). They are so called because their oxides and hydroxides are alkaline
in nature. The alkali metals are characterised by one s-electron and the alkaline earth
metals by two s-electrons in the valence shell of their atoms. These are highly reactive
metals forming monopositive (M
+
) and dipositve (M
2+
) ions respectively.
silicate (Ca
3
SiO
5
) 51% and tricalcium
aluminate (Ca
3
Al
2
O
6
) 11%.
Setting of Cement: When mixed with water,
the setting of cement takes place to give a hard
mass. This is due to the hydration of the
molecules of the constituents and their
rearrangement. The purpose of adding
gypsum is only to slow down the process of
setting of the cement so that it gets sufficiently
hardened.
Uses: Cement has become a commodity of
national necessity for any country next to iron
and steel. It is used in concrete and reinforced
concrete, in plastering and in the construction
of bridges, dams and buildings.
10.10 BIOLOGICAL IMPORTANCE OF
MAGNESIUM AND CALCIUM
An adult body contains about 25 g of Mg and
1200 g of Ca compared with only 5 g of iron
and 0.06 g of copper. The daily requirement
in the human body has been estimated to be
200 – 300 mg.
All enzymes that utilise ATP in phosphate
transfer require magnesium as the cofactor.
The main pigment for the absorption of light
in plants is chlorophyll which contains
magnesium. About 99 % of body calcium is
present in bones and teeth. It also plays
important roles in neuromuscular function,
interneuronal transmission, cell membrane
integrity and blood coagulation. The calcium
concentration in plasma is regulated at about
100 mgL
–1
. It is maintained by two hormones:
calcitonin and parathyroid hormone. Do you
know that bone is not an inert and unchanging
substance but is continuously being
solubilised and redeposited to the extent of
400 mg per day in man? All this calcium
passes through the plasma.
2019-20
313THE s-BLOCK ELEMENTS
EXERCISES
10.1 What are the common physical and chemical features of alkali metals ?
10.2 Discuss the general characteristics and gradation in properties of alkaline earth
metals.
10.3 Why are alkali metals not found in nature ?
10.4 Find out the oxidation state of sodium in Na
2
O
2
.
10.5 Explain why is sodium less reactive than potassium.
10.6 Compare the alkali metals and alkaline earth metals with respect to (i) ionisation
enthalpy (ii) basicity of oxides and (iii) solubility of hydroxides.
10.7 In what ways lithium shows similarities to magnesium in its chemical behaviour?
10.8 Explain why can alkali and alkaline earth metals not be obtained by chemical
reduction methods?
10.9 Why are potassium and caesium, rather than lithium used in photoelectric cells?
10.10 When an alkali metal dissolves in liquid ammonia the solution can acquire
different colours. Explain the reasons for this type of colour change.
10.11 Beryllium and magnesium do not give colour to flame whereas other alkaline
earth metals do so. Why ?
10.12 Discuss the various reactions that occur in the Solvay process.
10.13 Potassium carbonate cannot be prepared by Solvay process. Why ?
10.14 Why is Li
2
CO
3
decomposed at a lower temperature whereas Na
2
CO
3
at higher
temperature?
There is a regular trend in the physical and chemical properties of the alkali metal
with increasing atomic numbers. The atomic and ionic sizes increase and the ionization
enthalpies decrease systematically down the group. Somewhat similar trends are
observed among the properties of the alkaline earth metals.
The first element in each of these groups, lithium in Group 1 and beryllium in
Group 2 shows similarities in properties to the second member of the next group. Such
similarities are termed as the ‘diagonal relationship’ in the periodic table. As such
these elements are anomalous as far as their group characteristics are concerned.
The alkali metals are silvery white, soft and low melting. They are highly reactive.
The compounds of alkali metals are predominantly ionic. Their oxides and hydroxides
are soluble in water forming strong alkalies. Important compounds of sodium includes
sodium carbonate, sodium chloride, sodium hydroxide and sodium hydrogencarbonate.
Sodium hydroxide is manufactured by Castner-Kellner process and sodium carbonate
by Solvay process.
The chemistry of alkaline earth metals is very much like that of the alkali metals.
However, some differences arise because of reduced atomic and ionic sizes and increased
cationic charges in case of alkaline earth metals. Their oxides and hydroxides are less
basic than the alkali metal oxides and hydroxides. Industrially important compounds of
calcium include calcium oxide (lime), calcium hydroxide (slaked lime), calcium sulphate
(Plaster of Paris), calcium carbonate (limestone) and cement. Portland cement is an
important constructional material. It is manufactured by heating a pulverised mixture
of limestone and clay in a rotary kiln. The clinker thus obtained is mixed with some
gypsum (2-3%) to give a fine powder of cement. All these substances find variety of uses
in different areas.
Monovalent sodium and potassium ions and divalent magnesium and calcium ions
are found in large proportions in biological fluids. These ions perform important
biological functions such as maintenance of ion balance and nerve impulse conduction.
2019-20
314 CHEMISTRY
10.15 Compare the solubility and thermal stability of the following compounds of the
alkali metals with those of the alkaline earth metals. (a) Nitrates (b) Carbonates
(c) Sulphates.
10.16 Starting with sodium chloride how would you proceed to prepare (i) sodium metal
(ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate ?
10.17 What happens when (i) magnesium is burnt in air (ii) quick lime is heated with
silica (iii) chlorine reacts with slaked lime (iv) calcium nitrate is heated ?
10.18 Describe two important uses of each of the following : (i) caustic soda (ii) sodium
carbonate (iii) quicklime.
10.19 Draw the structure of (i) BeCl
2
(vapour) (ii) BeCl
2
(solid).
10.20 The hydroxides and carbonates of sodium and potassium are easily soluble in
water while the corresponding salts of magnesium and calcium are sparingly
soluble in water. Explain.
10.21 Describe the importance of the following : (i) limestone (ii) cement (iii) plaster of
paris.
10.22 Why are lithium salts commonly hydrated and those of the other alkali ions
usually anhydrous?
10.23 Why is LiF almost insoluble in water whereas LiCl soluble not only in water but
also in acetone ?
10.24 Explain the significance of sodium, potassium, magnesium and calcium in
biological fluids.
10.25 What happens when
(i) sodium metal is dropped in water ?
(ii) sodium metal is heated in free supply of air ?
(iii) sodium peroxide dissolves in water ?
10.26 Comment on each of the following observations:
(a) The mobilities of the alkali metal ions in aqueous solution are Li
+
< Na
+
< K
+
< Rb
+
< Cs
+
(b) Lithium is the only alkali metal to form a nitride directly.
(c) E
for M
2+
(aq) + 2e
–
→ M(s) (where M = Ca, Sr or Ba) is nearly constant.
10.27 State as to why
(a) a solution of Na
2
CO
3
is alkaline ?
(b) alkali metals are prepared by electrolysis of their fused chlorides ?
(c) sodium is found to be more useful than potassium ?
10.28 Write balanced equations for reactions between
(a) Na
2
O
2
and water
(b) KO
2
and water
(c) Na
2
O and CO
2
.
10.29 How would you explain the following observations?
(i) BeO is almost insoluble but BeSO
4
is soluble in water,
(ii) BaO is soluble but BaSO
4
is insoluble in water,
(iii) LiI is more soluble than KI in ethanol.
10.30 Which of the alkali metal is having least melting point ?
(a) Na (b) K (c) Rb (d) Cs
10.31 Which one of the following alkali metals gives hydrated salts ?
(a) Li (b) Na (c) K (d) Cs
10.32 Which one of the alkaline earth metal carbonates is thermally the most stable ?
(a) MgCO
3
(b) CaCO
3
(c) SrCO
3
(d) BaCO
3
2019-20
