(i) H₂O undergoes extensive H-bonding due to high electronegativity of O-atom and hence exist as an associated molecule.
(ii) Therefore H₂O is a liquid. On the other hand H₂O does not undergo H-bonding and hence exists as a discrete molecules and as a gas.

Nitrogen has smaller size and can polarise N-H bond d more efficiently than chlorine can do in a CI-H bond. Hence, nitrogen forms H- bond, while chlorine does not.

(i) l₂ < F2 < Br₂ < Cl₂·
(ii) HF < Hq < HBr < HI.

(i) Bleaching action of Cl₂ is due to its oxidising property.
(ii) In aqueous solution, Cl₂ liberates nascent oxygen which brings about the oxidation of coloured substances present in vegetables and organic matter to colourless substances.
\(\mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{HCl}+\mathrm{[O}]\)
Coloured substance +[O] → colorless substance.

(i) Sulphur has great catenation property and forms large molecule having high weight, which results higher melting point.
(ii) While oxygen exists as diatomic molecule of less molecular weight, hence has very low melting point.

(i) The electron gain enthalpy for oxygen is less negative because of its small size due to which the electron repulsions in the relatively small 2p-subshell are comparatively large.
(ii) Hence the Incoming electrons are not accepted with the same ease as in case of sulphur as it has relatively large size.

Element of group 15 have stable half-filled p-orbitals hence large amount of energy is required to remove electrons as compared to group 16, which has an incomplete p subshell.

(i) All the halogens have strong oxidising property due to their high electron affinity.
(ii) Fluorine is the strongest oxidising agent. It oxidises other halide ions into halogens in solution or when dry.
\({ F }_{ 2 }+{ 2X }^{ - }\longrightarrow { 2F }^{ - }+{ X }_{ 2 }\) (where X^- = Cl^-, Br^-, I^-)

Ammonia is prepared in the laboratory by heating an ammonium salt with a base.
\({ 2NH }_{ 4 }^{ + }+{ OH }^{ - }\longrightarrow { 2NH }_{ 3 }+{ H }_{ 2 }O\)
\({ 2NH }_{ 4 }Cl+CaO\longrightarrow { CaCl }_{ 2 }+{ 2NH }_{ 3 }+{ H }_{ 2 }O\)

(i) Nitrogen is used for the manufacture of ammonia, nitric acid and calcium Cyanamide etc.
(ii) Liquid nitrogen is used for producing low temperature required in cryosurgery and so in biological preservation.

(i) Nitrogen is separated industrially from liquid air by fractional distillation.
(ii) Pure nitrogen gas can be obtained by the thermal decomposition of sodium azide about 575K.
\(2Na{ N }_{ 3 }\longrightarrow 2Na+{ 3N }_{ 2 }\)

(i) sp³d³ hybridisation
(ii) Pentagonal bipyramidal structure.

(a) OF₂
+ 2 + 2(x) = 0 
+2 = -2x 
2 x = -2 ⇒ x = -1
(b) O₂F₂
2(+1) + 2x = 0 
2x = -2 
x = -1
(c) Cl₂O₃
2(x) + 3(-2) = 0 
2x = +6 
x = +3
(d) I₂O₄
2(x) + 4(-2) = 0 
2x = +8 
x = +4

(i) In heavier post transition metals, the outers electrons (ns) have a tendency to remain inert and show reluctance to take part in the bonding, which is known as inert pair effect.
(ii) This effect is also observed in groups 14, 15 and 16.

Each halogen combines with other halogens to form a series of compounds are called interhalogen compounds.
Example: AB type: BrF
AB₃ type: ICI₃