Question

X and Y are two metals. When burnt in air, X forms only oxide, while Y forms oxide and nitride. The metals X and Y may be:
(A) $\text{Ca}$ and $\text{Mg}$
(B) $\text{Na}$ and $\text{Mg}$
(C) $\text{Li}$ and $\text{Na}$
(D) $\text{Na}$ and $\text{K}$

Answer 1

The Reactivity of metals with air depends on the amount of the bond formation energy released after the formation of the product. The more energy released the more are the chances of the formation of the product.

Complete step by step answer:
Metals burn in air to react with oxygen and form oxides and with nitrogen to form nitrides. Alkali metals like sodium, potassium, etc. react with oxygen of the air to form the oxide. But these metals do not react with nitrogen because the lattice energy of most of the alkali metals, except lithium, is much less in comparison to the bond dissociation energy of the triple bond of nitrogen molecule required to form the nitride.
The nitride ion, ${{\text{N}}^{\text{3-}}}$ , is a highly negatively charged anion. Cations like magnesium ion, aluminium ion, and lithium ion to some extent can form strong ion bonds with nitride ions and release high lattice energy due to the small size of the cations and high charge density they exert high polarising effect on the nitride ion. Hence the nitrides formed are stable.
So, the correct answer is: $\text{Na}$ and $\text{Mg}$ , option B.

Note:
The formation of the nitride anion requires much more energy than the formation of the oxide anion and this is due to the triple bond between the nitrogen atoms while in the oxygen atoms, there are double bonds between the atoms.