Give reasons for the following:

(i) Oxygen is a gas but sulphur is solid.

(ii) O₃ acts as a powerful oxidising agent.

(iii) BiH₃ is the strongest reducing agent amongst all the hydrides of Group 15 elements.

(i) Oxygen forms O₂ which is a gas and sulphur forms S₈ which is solid this can be explained as:

Due to the small size of oxygen, it has less tendency for catenation and the high tendency of pp-pp multiple bonds, hence forms stable O₂ molecules whereas sulphur because of its higher tendency for catenation and lesser tendency to form pp-pp multiple bonds forms S₈ molecules having 8-membered puckered ring. Held together by strong covalent bonds and exist as a polyatomic molecule, so it exists solid.

(ii) Ozone is not a very stable compound under normal conditions and decomposes readily on heating to give a molecule of oxygen and nascent oxygen. Nascent oxygen, being a free radical, is very reactive.

Therefore, O₃ acts as powerful oxidising agent.

(iii) BiH₃ is the strongest reducing agent amongst all the hydrides of group-15 elements because as we more down the group, the atomic size increases and the stability of the hydrides of group 15 element decreases. Since the stability of hydrides decreases on moving from NH₃ to BiH₃, the reducing character of the hydrides increases on moving from NH₃ to BiH₃.