(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) The two O-O bond lengths are identical due to resonance.
(ii) Due to the strong bond formation with other elements.

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

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) Nitric acid is used as a oxidising agent and in the preparation of aqua regia.
(ii) Salts of nitric acid are used in photography (AgNO₃) and gun powder for fire arms (NaNO₃)·

(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.

(i) Fluorine is more electro negative than other halogens.
(ii) Because except fluorine all the other halogens have positive oxidation state.
(iii) It has high electron affinity character
Example:
\(\mathrm{Cl}_{2}\mathrm{O} \Rightarrow+1 \text { oxidation state }(\mathrm{Cl}) \)
\(\mathrm{OF}_{2} \Rightarrow-1 \text { oxidation state }(\mathrm{F})\)

(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) Fluorine is most electronegative atom.
(ii) It has only one unpaired electron.

(i) The Chalcogens belong to group (16).
(ii) The group consists of elements: Oxygen, Sulphur, Selenium, Tellurium and Polonium.
(iii) These are ore forming elements as most of the ores are oxides and sulphides.
(iv) Chalcos meaning 'ore formers'.

(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.

(i) Helium is used to provide inert atmosphere in electric-arc welding of metals.
(ii) Helium has lowest boiling point hence used in cryogenics.
(iii) It is much less denser than air and hence used for filling air balloons.

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