11 Chemistry Notes - The S-Block Elements
11 Chemistry Notes - The S-Block Elements
11 Chemistry Notes - The S-Block Elements
Subject: Chemistry
Class: XI
1. The s-block elements of the periodic table are those in which the last
electron enters the outermost s-orbital
2. Elements of group1 are called alkali metals and elements of group 2 are
called alkaline earth metal
(i) Group 1 elements are called alkali metals because they form hydroxides
on reaction with water which are strongly alkaline in nature
1
(ii) The general electronic for alkali metals is [noble gas] ns
configuration
1
Lithium Li 3
[He]2s
1
Sodium Na 11
[Ne]3s
1
Potassium K 19
[Ar]4s
1
Rubidium Rb 37
[Kr]5s
Caesium Cs 55 1
[Xe]6s
1
Francium Fr 87
[Rn]7s
1. Atomic and ionic radii: Alkali metals have the largest atomic and ionic radii
in their respective periods of the periodic table. On moving down the group,
the atomic and ionic radii increase
4. Melting and boiling points: Alkali metals are soft and have low melting
and boiling points
Explanation: Alkali metals have only one valence electron per metal atom
and therefore, the energy binding the atoms in the crystal lattice of the metal
is low. Thus, the metallic bonds in these metals are not very strong and
consequently, their melting and boiling points decrease on moving down
from Li to Cs
Explanation: The densities of metallic elements depend upon the type of packing
of atoms in metallic state and also on their size. The alkali metals have close
packing of metal atoms in their lattice and because of the large size of their
atoms, they have low densities. On moving down the group from Li to Cs, there
is increase in atomic size as well as atomic mass. But the increase in atomic
mass is more and compensates the increase in atomic size. As a result, the
densities (mass/volume) of alkali metals gradually increase from Li to Cs. K is
lighter than Na due to increase in atomic size of K
As alkali metals have low ionization enthalpies, their atoms readily lose their
valence electron. These elements are, therefore, said to have strong
electropositive or metallic character. Since, the ionization energies decrease
down the group, the electron releasing tendency or electropositive
character is expected to increase down the group
Explanation: Alkali metals have only one electron in their valence shell and
therefore can lose the single valence electron readily to acquire the stable
configuration of a noble gas. Since the second ionization energies are very
high, they cannot form divalent ions. Thus, alkali metals are univalent and
form ionic compounds
8. Characteristic flame colouration: All the alkali metals and their salts impart
characteristic flame colouration
Explanation: Alkali metals have very low ionization enthalpies. The energy
from the flame of Bunsen burner is sufficient to excite the electrons of alkali
metals to higher energy levels. Excited state is quite unstable and therefore
when these excited electrons come back to their original energy levels, they
emit extra energy, which falls in the visible region of the electromagnetic
spectrum and thus appear coloured. Characteristic flame colouration by
different alkali metals can be explained on the basis of difference in amount of
energy absorbed for excitation of the valence electron
Explanation: Alkali metals have low ionization enthalpies and therefore, the
electrons are easily ejected when exposed to light. Among alkali metals,
caesium has lowest ionization enthalpy and hence it shows photoelectric
effect to the maximum extent
10. Hydration of alkali metal ions: Alkali metal ions are highly hydrated.
Explanation: Smaller the size of the ion, the greater is the degree of hydration.
+ +
Thus, Li ion gets much more hydrated than Na ion which is more hydrated
+ +
than K ion and so on. Therefore, the extent of hydration decreases from Li to
+ + +
Cs . As a result of larger hydration of Li ion than Na ion, the effective size of
+ +
hydrated Li ion is more than that of hydrated Na ion.
Hydrated ionic radii decrease in the order: Li+ > Na+ > K +
> Rb
+
> Cs
+
+
Due to extensive hydration, Li ion has lowest mobility in water.
11. Reducing Nature: Alkali metals are strong reducing agents. This is due
to their greater ease to lose electrons
Lithium being small in size has high ionization enthalpy. On the other hand
because of small size it is extensively hydrated and has very high hydration
enthalpy. This high hydration enthalpy compensates the high energy needed
to remove electron (in second step). Thus Li has greater tendency to lose
electrons in solution than other alkali metals. Thus Li is the strongest
reducing agent.
Explanation: In solution the alkali metal atom readily loses the valence
electron. Both the cation and the electron combine with ammonia to form
ammoniated cation and ammoniated electron. 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
Sodium
Solvay Process:
Carbonate
ammonia, sodium
bicarbonate is formed.
Sodium bicarbonate on
carbonate
impurities of CaSO4 ,
dissolved in minimum
filtered to remove
insoluble impurities.
Crystals of pure
sodium
Electrolysis of NaCl in
Sodium
Castner- Kellner cell:
Hydroxide
A brine solution is
(Caustic Soda),
electrolysed using a
NaOH
mercury cathode and a
carbon anode. Na
metal
discharged at the
mercury to form
sodium amalgam.
amalgam on treatment
Hydrogencarbo
n saturating a solution of
(i) Group 2 elements are called alkaline earth metals because their oxides
and hydroxides are alkaline in nature and these metal oxides are found in
the earths crust
(ii) The general electronic configuration for alkaline earth metals is [noble
2
gas] ns
configuration
Beryllium Be 4 [He]2s2
Magnesium Mg 12 [Ne]3s2
Calcium Ca 20 [Ar]4s2
Strontium Sr 38 [Kr]5s2
Barium Ba 56 [Xe]6s2
6
Radium Ra 88 [Rn]
7s
1. Atomic and ionic radii: Atomic and ionic radii of alkaline earth metals
increases down the group and are smaller than the corresponding
members of the alkali metals
Explanation: Although IE1 values of alkaline earth metals are higher than
those of alkali metals, the IE2 values of alkaline earth metals are much
smaller than those of alkali metals
In case of alkali metals (for e.g. Na) the second electron is to be removed
from a cation which has already acquired a noble gas configuration.
However in the alkaline earth metals (for e.g. Mg), the second electron is to
+ 2 2 6 1
be removed from a monovalent cation, i.e. Mg : (1s 2s p 3s ) which still
has one electron in the outermost shell. Thus, the second electron in Mg can
be removed easily.
3. Melting and boiling points: Alkaline earth metals have higher melting
and boiling points than the corresponding alkali metals
5. Oxidation states: All the alkaline earth metal exhibits an oxidation state of
+2 in their compounds
(ii)In solution, the +2 ions of alkaline earth metals are extensively hydrated
2+ +
and the high hydration energies of E ions make then more stable than E
2+
ions. It is observed that the amount of energy released when E ion is
+
dissolved in water is much more that that for E ions. This large amount of
extra energy released in the hydration of +2 ions is more and compensates the
2+
second ionization enthalpy required for the formation of such E ions
(iii)In the solid state, the divalent cations form stronger lattices than
monovalent cations and therefore, a lot of energy called lattice enthalpy is
2+
released. It is the greater lattice enthalpy of E ion which compensates for
the high second ionization enthalpy and is responsible for its greater stability
+
as compared to E ion
2+ 2+ 2+ 2+ 2+
Be > Mg > Ca > Sr > Ba
It is prepared by heating
Calcium Oxide
limestone (CaCO3) in a
or Quick
Lime,
rotary kiln at 1070-1270 K.
CaO
The carbon dioxide is
removed as soon as it is
reaction to proceed to
completion
8
(Slaked lime),
Ca(OH)2
It is prepared by passing
Calcium
carbon dioxide through
Carbonate,
slaked lime. Excess of
CaCO3
carbon dioxide should be
soluble calcium
hydrogencarbonate
It is prepared by the
addition of sodium
carbonate to calcium
chloride.
It is obtained when
Calcium gypsum,
Paris),
CaSO4·½ H2O
5. Diagonal relationship: It is observed that some elements of second period
show similarities with the elements of third period present diagonally to each
other though belonging to different groups. This is called diagonal
relationship
(3) Alkali and alkaline earth metals cannot be obtained by chemical reduction
methods
Explanation: Alkali and alkaline earth metals are themselves very strong
reducing agents and therefore cannot be reduced by chemical reduction
methods
Bicarbonate
s
-
-
H HCO +
CO + O OH
3 2 3
Carbonates
Explanation: According to Fajans rule smaller the size of cation and larger
the size of anion greater is the covalent character of ionic bond. Li is small in
+ +
size than K, thus Li has a high charge density. Thus polarizing power of Li
+
is higher than K , hence LiCl is more covalent than KCl.
2- 2-
Explanation: Size of O ion is smaller than SO4 . Since a bigger anions
stabilizes bigger cation more than a smaller cation stabilizes a bigger anion,
10
Explanation: 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.
Explanation: The M-OH bond in hydroxides of alkali metal is very weak and
+ -
can easily ionize to form M ions and OH ions. This accounts for the basic
character. Since ionization energy decreases down the group bond
between metal and oxygen becomes weak. Therefore basic strength of
hydroxides increases accordingly. Thus NaOH is a stronger base than LiOH
(12) Except for Be and Mg, the alkaline earth metal salts impart
characteristic colours to the flame