Question

Account for the following:
(i) Helium is used in diving apparatus.
(ii) Flourine does not exhibit positive oxidation state.
(iii) Oxygen shows catenation behaviour less than sulphur.

Answer 1

i. The main reason to use helium in the deep sea diving breathing mix (Heliox, 0% $N_2$) is to reduce the proportions of nitrogen and oxygen in a standard Nitrogen/Oxygen mix (Nitrox) as the lower proportion of nitrogen reduce the chances of "nitrogen narcosis" in deep phase of diving and also the lower proportion of oxygen.
ii. Fluorine is very small in size and the most electronegative element so it accepts electrons very easily to complete its octet. That's why it does not have positive oxidation state.

ii. Oxygen shows catenation behaviour less than sulphur because the oxygen atom is smaller in size as compared to sulphur due to this the lone pair of electrons in O-O bonds in oxygen experiences more repulsions as compared to the S-S bonds and therefore, S-S forms strong bond.