(i) Bond angel of HiX (H-O-H=104.5^o) is larger than that of H₂S(H-S-H=92^o)
(ii) Since oxygen is more electronegative than sulphur, therefore bond pair electrons of O-H bond will be closer to oxygen which will be little away from the sulphur atom.
(iii) As a result bond pair-bond pair repulsions between bond pairs of two O-H bonds would be stronger than in the S-H bonds.

(i) The thermal stability of hydrides of group 16 elements is directly proportional to the bond dissociation enthalpy of H- E bond.
(ii) On moving down the group, bond dissociation energy decreases and hence, E- H bond breaks easily.
(iii) Thus, the thermal stability of hydrides of group 16 elements decreases down the group. Hence the order of thermal stability is,
H₂O > H₂S > H₂Se > H₂Te > H₂Po

(i) Fluorine is the most reactive element among halogens due to minimum value of F- F bond dissociation energy.
(ii) It can form two types of salts with metals NaF and NaHF₂
(iii) AgF is soluble in water but other AgX are insoluble.
(iv) HF attacks glass while others do not.
(v) Fluorine, does not form any polyhalides (absence of d - orbitals)
(vi) Fluorine exhibit only negative oxidation state (highly electronegative) while other halogens have both +ve and -ve oxidation state.

(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

Yellow phosphorus reacts with alkali on boiling in an inert atmosphere liberating phosphine. Phosphorus acts as a reducing agent.
\({ P }_{ 4 }+NaOH+{ H }_{ 2 }O\longrightarrow \underset { Sodiumhypophosphite }{ { 3NaH }_{ 2 }{ PO }_{ 2 } } +\underset { Phosphine }{ { PH }_{ 3 } } \uparrow \)

White Phosphorous glows in the dark due to oxidation which is called as phosphorescence.

(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.
\(2Na{ N }_{ 3 }\longrightarrow 2Na+{ 3N }_{ 2 }\)

\((i) \quad 4 \mathrm{NaCl}+\mathrm{MnO}_{2}+4 \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{Cl}_{2}+\mathrm{MnCl}_{2}+4 \mathrm{NaHSO}_{4}+2 \mathrm{H}_{2} \mathrm{O} \)
\((ii) \quad \mathrm{NaNO}_{2}+\mathrm{HCl} \rightarrow \mathrm{NaCl}+\mathrm{HNO}_{2} \)
\((iii) \quad \mathrm{P}_{4}+3 \mathrm{NaOH}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow 3 \mathrm{NaH}_{2} \mathrm{PO}_{2}+\mathrm{PH}_{3} \uparrow \)
\((iv) \quad 3 \mathrm{AgNO}_{3}+\mathrm{PH}_{3} \rightarrow \mathrm{Ag}_{3} \mathrm{P}+3 \mathrm{HNO}_{3} \)
\((v) \quad 4 \mathrm{Mg}+10 \mathrm{HNO}_{3} \rightarrow 4 \mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}+\mathrm{N}_{2} \mathrm{O}+6 \mathrm{H}_{2} \mathrm{O} \)
\((vi) \quad 2 \mathrm{KClO}_{3} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{KCl}+3 \mathrm{O}_{2} \uparrow \)
\((vii) \quad \mathrm{Cu}+Con. Hot \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{CuSO}_{4}+2 \mathrm{H}_{2} \mathrm{O}+\mathrm{SO}_{2} \uparrow \)
\((viii) \quad 2 \mathrm{Sb}+3 \mathrm{Cl}_{2} \rightarrow 2 \mathrm{SbCl}_{3} \)
\((ix) \quad 2 \mathrm{HBr}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow 2 \mathrm{SO}_{2}+2 \mathrm{H}_{2} \mathrm{O}+\mathrm{Br}_{2} \)
\((x) \quad \mathrm{XeF}_{6}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{XeO}_{3}+6 \mathrm{HF} \)
\((xi) \quad 5 \mathrm{XeO}_{6}^{4-}+2 \mathrm{Mn}^{2+}+14 \mathrm{H}^{+} \rightarrow 2 \mathrm{MnO}_{4}^{-}+5 \mathrm{XeO}_{5}+7 \mathrm{H}_{2} \mathrm{O} \)
\((xii) \quad 2 \mathrm{XeOF}_{4}+\mathrm{SiO}_{2} \rightarrow 2 \mathrm{XeO}_{2} \mathrm{~F}_{2}+\mathrm{SiF}_{4} \)
\((xiii) \quad Xe+{ 3F }_{ 2 }\overset { Ni/200atm }{ \underset { 400^{ 0 }C }{ \longrightarrow } }XeF_6\)

(i) The general electronic configuration of 15 group elements is ns²np³.
(ii) Nitrogen \(\Rightarrow 2 \mathrm{~s}^{2} 2 \mathrm{p}^{3}\) (Valence shell)
(iii) Phosphorus \(\Rightarrow 3 \mathrm{~s}^{2} 3 \mathrm{p}^{3}\)
(iv) Arsenic \(\Rightarrow 4 \mathrm{~s}^{2} 4 \mathrm{p}^{3}\)
(v) Antimony \(\Rightarrow 5 s^{2} 5 p^{3}\)
(vi) Bismuth \(\Rightarrow 6 \mathrm{~s}^{2} 6 \mathrm{p}^{3}\)

Reaction with metals:
(i) Sulphuric acid reacts with metals and gives different product depending on the reactants and reacting condition
(ii) Dilute sulphuric acid reacts with metals like: tin, aluminium, zinc to give corresponding: sulphates.
Zn + H₂SO₄\(\longrightarrow \) ZnSO₄ + H₂ \(\uparrow \)
2AI + 3H₂SO₄ \(\longrightarrow \) Al₂(SO₄)₃+ 3H₂ \(\uparrow \)
(iii) Hot concentrated sulphuric acid reacts with copper and lead to give the respective sulphates as shown below 
Cu + 2H₂SO₄ \(\longrightarrow \) CuSO₄ + 2H₂O + SO₂\(\uparrow \)
Pb + 2H₂SO₄ \(\longrightarrow \) PbSO₄ + 2H₂O + SO₂\(\uparrow \)
(iv) Sulphuric acid doesn't react with noble metals like gold, silver and platinum.

(a) Rhombic Sulphur (α - Sulphur):
(a) It is yellow in colour.
(b) Its melting point is 385.8K and specific gravity is 2.06
(c) It is stable form of sulphur at room temperature.
(d) It is formed on evaporating the solution of sulphur in CS₂.
(e) It in insoluble in water, readily soluble in CS₂ and dissolves to some extent in benzene, alcohol and ether.
(b) Monoclinic sulphur \(\left( \beta -sulphur \right) \):
(a) Its melting point is 393K and specific gravity is 1.98
(b) It is prepared by melting rhombic sulphur in a dish and cooling, till crust is formed. Two holes are made in crust and remaining liquid is powered out. On removing crust, colourless needle - shaped crystals of β - sulphur is formed.
(c) Monoclinic sulphur is stable above 369K and below 369K α - sulphur is stable.
(d) At 369K both forms are stable and this temperature is called transition temperature.
(e) Both rhombic and monoclinic sulphur have S₈ molecules, these are packed to give different crystal structure S₈ form is puckered and has crown shape.
Several other modifications containing 6-20 sulphur atoms per ring are synthesised 

(f) In Cyclo-S₆ the ng adopts chair form.

(g) At elevated temperatures (～1000K), S₂ is dominant species and is, paramagnetic like O₂