Question

Assertion: Dinegative anion of oxygen $({{O}^{2-}})$ is quite common but Negative anion of sulfur $({{S}^{2-}})$ is less common.
Reason: Covalency of oxygen is two.
(A)- If both assertion and reason are true and the reason is a correct explanation of the assertion.
(B)- If both assertion and reason are true but the reason is not a correct explanation of the assertion.
(C)- If the assertion is true and the reason is false
(D)- If the assertion is false but the reason is true

Answer 1

The group 16 elements have a decrease in the electronegativity down the group, with the increase in size of the atom. Also, being close to the halogen group, which is highly electronegative in nature and gains electrons easily to attain stability, this group also shows a slightly similar behaviour.

Complete step by step answer:
Both the elements, oxygen and sulfur belong to the group 16 of the periodic table. They have the electronic configuration of $n{{s}^{2}}n{{p}^{4}}$ . Due to the lack of only two electrons in the valence shell, in order to attain the stable configuration, it has a tendency to gain two electrons.
But due to the small size and absence of d-orbitals in the oxygen atom it shows $(-2)$ oxidation state only. Whereas, in sulfur it also shows $(+6,+4)$ oxidation state as well. This is because the oxygen being highly electronegative in nature, easily gains two electrons showing a covalency equal to 2.
This electronegative tendency decreases in sulfur as its size increases, with the addition of a new n - shell. So, the valence electrons in it get further away from the nucleus and due to shielding effect by the non-valence electrons, they are less attracted by the nucleus.

Note: Though the electronegativity of the oxygen is more than sulfur, the electron affinity (that is, the amount of energy released when an electron is added) is less compared to sulfur. This is because the oxygen being small in size, the electrons face significant repulsion which makes it unstable. In order to prevent this, during the addition of electrons, some energy is absorbed to overcome this instability. Thus, lowering its electron affinity than sulfur.