(i) In p-block elements, the first member of each group differs from the other elements of the corresponding group. The following factors are responsible for this anomalous behaviour.
(ii) Small size of the first member.
(iii) High ionisation enthalpy and high electronegativity.
(iv) Absence of d-orbitals in their valence shell.

1. A double salt which contains fourth-period alkali metal (A) is potash alum
K₂SO₄ Al₂(SO₄)₃ - 24 H₂O
2. On heating potash alum (A) 500 k give anhydrous potash alum (or) burnt alum (B).
\(\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O} \stackrel{500 \mathrm{~K}}{\longrightarrow} \mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}+24 \mathrm{H}_{2} \mathrm{O}\)
[Potash alum (A)]                         [Burnt alum (B)]
3. Aqueous solution of burnt alum, has sulphates ion, potassium ion and aluminium ion. Sulphate ion reacts with BaCl₂ to form a white precipitate of Barium Sulphate
(SO₄)₂ + BaCl₂ → BaSO₄ + 2 Cl¯
Aluminium ion reacts with alizarin solution to give a red colour compound.

A hydride of 2^nd period alkali metal (A) is lithium hydride (LiH).
Lithium hydride (A) reacts with diborane (B) to give lithium borohydride (C) which is acts as a reducing agent.
B₂H₆ + 2 LiH \(\xrightarrow[]{ether}\) 2 LiBH₄
[Diborane (B)]   [Lithium hydride (A)]  [Lithium borohydride (C)]
Result:

Compound	Formula	Name
A	LiH	Lithium hydride
B	B2H6	Diborane
C	LiBH4	Lithium borohydride

Fusion of urea with B(OH)_3' in an atmosphere of ammonia at 800 - 1200 K gives boron nitride.
B(OH)₃ + NH₃ \(\overset { \Delta }{ \longrightarrow } \) BN+ 3H₂O

Generally on descending a group the ionisation energy decreases and hence the metallic character increases.

(i) Silicones are used for low temperature lubrication and in vacuum pumps, high temperature oil baths etc.
(ii) They are used for making water proofing clothes.
(iii) They are used as insulting material in electrical motor and other appliances.
(iv) They are mixed with paints and enamels to make them resistant towards high temperature, sunlight, dampness and chemicals.

(i) Borax is used for the identification of coloured metal ions.
(ii) In the manufacture optical and borosilicate glass, enamels and glazes for pottery.
(iii) It is also used as a flux in metallurgy.
(iv) It acts as a good preservative.