(a) Give reasons for the following: (i) Bond enthalpy of ${F}_2$ is lower than that of ${Cl}_2$. (ii) ${PH}_3$, has lower boiling point than ${NH}_3$. (b) Draw the structures of the following molecules: (i) ${BrF}_3$ (ii) $({HPO}_3)_3$ (iii) ${XeF}_4$ OR (a) Account for the following: (i) Helium is used in diving apparatus. (ii) Fluorine does not exhibit positive oxidation state. (iii) Oxygen shows catenation behaviour less than sulphur. (b) Draw the structures of the following molecules: (i) $XeF_2$ (ii) ${H}_2 {S}_2 {O}_8$