Explain anomalous behaviour of oxygen with respect to(i) Atomicity(ii) Magnetic property(iii) Oxidation state(iv) Nature of hydride(b) Write any two uses of helium.

Answer: Oxygen differs from other group 16 elements because of its small size, high electronegativity and absence of d orbitals.

(a) (i) Atomicity: oxygen forms pπ–pπ double bonds and exists as diatomic O₂ gas, while sulphur and others are polyatomic solids (S₈).

(ii) Magnetic property: O₂ is paramagnetic (two unpaired electrons), while the other elements (S₈ etc.) are diamagnetic.

(iii) Oxidation state: oxygen shows mainly –2 (–1 in peroxides, +2 in OF₂) and cannot show +4 or +6 because it has no d orbitals; S, Se, Te show +2, +4 and +6.

(iv) Nature of hydride: H₂O is a liquid at room temperature because of intermolecular hydrogen bonding and is neutral, while H₂S, H₂Se, H₂Te are gases and weakly acidic.

(b) Uses of helium (any two): filling balloons and airships (light and non-inflammable); He–O₂ mixture for breathing by deep-sea divers (helium is less soluble in blood than N₂); liquid helium as a cryogenic coolant, e.g. for superconducting magnets in MRI; inert atmosphere in welding and metallurgy.