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

(i) HCl < HBr < HI < HF
(ii) Fluorine being the most electronegative element cannot have positive oxidation state. Other halogens have d-orbitals, therefore can expand their octet.

(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^-)

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

(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 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.

a) BrF₅

Valence electron of bromine atom 7+ Number of fluorine atom (5) = 12
\(X=\frac{12}{2}=6\) 
Hybridization: sp³d² ;
Geometry: Square Pyramidal
b) BrF₃

Valence electron of bromine atom 7+ Number of fluorine atom (3) = 10 
X = \(\frac{10}{2}=5\)
Hybridization: sp³d²;
Geometry: Triangular bipyramidal (T - shaped)

HF is a weak acid i.e. 0.1 M solution is only 10% ionised, but in 5M & 15M solution, HF is stronger acid due to chemical equilibrium.
\(\mathrm{HF}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{F}^{-}  \)
\(\mathrm{HF}+\mathrm{F}^{-} \rightleftharpoons \mathrm{HF}_{2}^{-}\)

Argon prevents the oxidation of hot filament and prolongs the life in filament bulbs.

\(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow 12 \mathrm{C}+\mathrm{H}_{2} \mathrm{SO}_{4} \cdot 11 \mathrm{H}_{2} \mathrm{O} \\ \text { Sucrose } \)
\(\underset{\text { Formic acid }}{\mathrm{HCOOH}}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{CO}+\mathrm{H}_{2} \mathrm{SO}_{4} \cdot \mathrm{H}_{2} \mathrm{O}\)
\(\underset{\text { Oxalic acid }}{\mathrm{(COOH)}_{2}}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{CO}+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{SO}_{4} \cdot \mathrm{H}_{2} \mathrm{O}\)

Chlorine is prepared by the action of conc. sulphuric acid on chlorides in presence of manganese dioxide
4NaCl + MnO₂ + 4H₂SO₄ \(\longrightarrow \)Cl₂+ MnCl₂ +4NaHSO₄ + 2H₂O

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

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

(l) Cl₂+ H₂O\(\longrightarrow \) HCI + HOCI
(ii) XeF₆ + 2H₂O \(\longrightarrow \) XeO₃ + 6HF
(Iii) XeF₆ + 2H₂O\(\longrightarrow \) XeO₂F₂+ 4HF

(i) CI has vacant d-orbitals and hence can show an oxidation state of +3 but F has no d-orbitals, therefore, it cannot show positive oxidation states. Further, since F can show only - 1 oxidation state therefore it forms only CIF of FCI₃.
(ii) Due to larger size CI an accommodate three small F-atoms around it while F being smaller cannot accommodate three bigger sized CI atoms around it.

(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) Both fluorine and oxygen have very high electron affinities and can easily cause the excitation of the electrons from 5p orbital to 5d orbital of xenon.
(ii) The unpaired electrons thus formed can take up electrons from oxygen or fluorine to form compounds.
(iii) Thus xenon has low ionisation energy and it can form compounds with strong oxidising agents (high E.A) like F₂ and O₂·

O₃ oxidises potassium iodide to iodine. This reaction is quantitative and can be used for estimation of ozone 
\({ O }_{ 3 }+2KI+{ H }_{ 2 }O\longrightarrow 2KOH+{ O }_{ 2 }+{ I }_{ 2 }\)

Direct reaction of nitrogen with hydrogen gives ammonia. This reaction is favored by high pressures and at optimum temperature in presence of iron catalyst.
\(\cfrac { 1 }{ 2 } { N }_{ 2 }+\cfrac { 3 }{ 2 } { H }_{ 2 }\rightleftharpoons { 2NH }_{ 3 }\)\(\Delta H_f { }_{ }=-46.2KJmol^{ -1 }\) 

(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

(i) As per the position in the periodic table, the element A is phosphorus as below:
(ii) Phosphorus reacts with dry chlorine to form phosphorus trichloride as below
\(\underset { (A) }{ { P }_{ 4 } } +6{ Cl }_{ 2 }\longrightarrow \underset { (B) }{ { 4PCl }_{ 3 } } \)
Hence, compound B is phosphorus trichloride.
(iii) PCl₃ on hydrolysis gives phosphorus acid H₃PO₃, which is dibasic acid,
\(\underset { (B) }{ { PCl }_{ 3 } } +3{ H }_{ 2 }O\longrightarrow { 3HCl }+\underset { (C) }{ { H }_{ 3 }{ PO }_{ 3 } } \)  
Hence, compound C is phosphorus acid.
(iv) H₃PO₃ on heating undergo auto oxidation and reduction producing phosphoric acid md phosphine as below:
\(\underset { (c) }{ { 4H }_{ 3 }{ PO }_{ 3 } } \overset { \Delta }{ \longrightarrow } { PH }_{ 3 }+\underset { (b) }{ { 3H }_{ 3 }{ PO }_{ 4 } } \)
Phosphoric acid is a tribasic acid. Hence, compound D is phosphoric acid.
Thus A = Phosphorus
B = Phosphorus trichloride
C = Phosphorus acid
D = Phosphoric acid

(i) The element which occupies group number 15 and period number 3 is phosphorus. Hence, A is phosphorus This is further supported by the fact that A exhibits allotropy and A is tetra atomic.
(ii) Phosphorus reacts with caustic soda to form phosphine as below:
4P + 3 NaOH + 3H₂O \(\longrightarrow \) PH₃ + 3 NaH₂PO₂
Hence, B is phosphine PH₃ which has rotten fish odour.
(iii) Phosphorus reacts with dry chlorine to form phosphorus trichloride which has a pungent odour.
P₄ + 6 Cl₂\(\longrightarrow \) 4PCl₃
Thus, A = Phosphorus
B = Phosphine
C = Phosphorus trichloride

(I) Since the byproduct of roasting to sulphide ore is SO₂ It turns lime water milky.
Therefore, gas 'B' must be SO₂
(ii) As the gas 'B' is obtained when amorphous solid 'A' burns in air therefore, amorphous solid 'A' must be sulphur S₈
\(\underset { (A) }{ { S }_{ g } } +{ 8O }_{ 2 }\overset { \Delta }{ \longrightarrow } \underset { (B) }{ { 8SO }_{ 2 } } \)
(iii) Gas (B) reduces acidified aqueous KMnO₄ solution and reduces Fe³⁺ to Fe²⁺ salts as shown below:
\(\underset { (yellow) }{ { 2MnO }_{ 4 } } ^{ - }+\underset { (b) }{ { SO }_{ 2 } } +2{ H }_{ 2 }O\longrightarrow { 2Fe }^{ 2+ }+\underset { (Green) }{ { SO }_{ 4 }^{ 2- } } +{ 4H }^{ + }\)
(iv) Thus, solid 'A' is S₈ and gas 'B' is SO₂

(i) O₂ molecules are held together by weak Vander Waal's force because of small size and high electronegativity of oxygen.
(ii) In contrast, sulphur shows catenation and forms stronger S-S bonds.
(iii) Due to catenation, sulphur forms octa-atomic S₈ molecules having eight membered puckered ring structure.
(iv) Because of its bigger size the force of attraction holding S₈ molecules are much stronger.
(v) Hence sulphur is a solid at room temperature or in other words, that is why there is a large difference between the boiling point (also melting points) of the two elements.

(i) Because of its lightness and noninflammability helium is used to filling balloons for meteorological observations.
(ii) Because of its lightness it is used in inflating aeroplane tyres.
(iii) Helium oxygen mixture is used by deep sea divers in preference to nitrogen oxygen mixtures. This prevents bends when a diver comes to the surface.
(iv) A mixture of oxygen and helium is used in the treatment of asthma.
(v) Liquid helium (b.pt 4.2K) is used as cryogenic agent for carrying out various experiments at low temperatures.
(vi)It is used to produce and sustain powerful super conducting magnets of modern NMR Spectrometers and Magnetic Resonance Imaging system (MRI) for clinical diagnosis.