(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_2S_2O_8`

Explanation

(a) (i) Helium mixed with oxygen pressure is given to sea-divers for respiration. Air is not given to sea-divers because nitrogen present in air being soluble in blood will give a painful sensation called bends by bubbling out blood on moving from high pressure to the atmospheric pressure.
(ii) Fluorine being the most electronegative atom does not exhibit positive oxidation state because the electrons in fluorine are strongly attracted by the nuclear charge because of small size of fluorine atom and therefore, removal of an electron is not possible.
(iii) See Q. 23 (ii), Set-I, D.B., 2009.
(b) (i) See Q. 4, Set-I, D.B., 2011.
(ii) (a) (i) `H_2 S_2 O_8` (Peroxodisulphuric acid)