(a) Account for the following: (i) Acidic character increases from $\mathrm{HF}$ to $\mathrm{HI}$. (ii) There is large difference between the melting and boiling points of oxygen and sulphur. (iii) Nitrogen does not form pentahalide. (b) Draw the structures of the following: (i) $\mathrm{C}/\mathrm{F}_3$ (ii) $\mathrm{XeF}_4$ OR (i) Which allotrope of phosphorus is more reactive and why? (ii) How the supersonic jet aeroplanes are responsible for the depletion of ozone layers? (iii) $\mathrm{F}_2$ has lower bond dissociation enthalpy than $\mathrm{Cl}_2$. Why? (iv) Which noble gas is used in filling balloons for meteorological observations ? (v) Complete the equation: $\mathrm{XeF}_2+\mathrm{PF}_5 \rightarrow$