Question

Which of the following alkali metals when burnt in air forms a mixture of oxide as well as nitride?
(A) $K$
(B) $Na$
(C) $Li$
(D) $Cs$

Answer 1

Lithium when burnt in air forms a mixture of oxide as well as nitride $4Li + {O_2}\xrightarrow{\Delta }2L{i_2}O,{\text{ }}6Li + {N_2}\xrightarrow{\Delta }2L{i_3}N$

Complete step by step answer:
The alkali metals tarnish in dry air due to formation of their oxides which in turn react with moisture to form hydroxide. They burn vigorously in oxygen forming oxides. Lithium forming monoxide $L{i_2}O$, sodium forms peroxide $N{a_2}{O_2}$, the other metals form ${O^{2 - }}$ superoxide ions
$4Li + {O_2} \to 2L{i_2}O$ (Normal oxide)
$2Na + {O_2} \to 2N{a_2}{O_2}$ (peroxide)
$M + {O_2} \to M{O_2}$ (Superoxide)
Where $M$ is $M = K,Rb,Cs$
In all these oxides the oxidation state of the alkali metal is $+ 1$. Lithium shows exceptional behaviour in reacting directly with Nitrogen of air to form Nitride, $L{i_3}N$.
The reactions are as follow:
$4Li + {o_2} \to 2L{i_2}O$
$6Li + {N_2} \to 2L{i_3}N$
Hence, the answer is option (C).

Additional information:
Lithium is an alkali metal but the properties of lithium are different than other alkali metals.
1. Lithium is harder than other alkali metals
2. Melting and boiling point is higher than other alkali metals.
3. Out of all other alkali metals, it is least reactive alkali metal.
4. It is a strong reducing agent compared to other alkali metals.
5. It is only alkali metal forming monoxide.
6. Compared to other alkali metals, it is not capable of forming solid hydrogen carbonates.
7. It is only alkali metals which reacts with Nitrogen to form Nitride.
8. It does not react with ethyne to form ethynide. On the other hand, all other alkali metals form ethynide.
9. It reacts slowly with Bromine as compared to other alkali metals.

Note:
The polarization behavior of the ions of Lithium is somewhat similar to that of Magnesium. Moreover, many properties of Lithium are similar to Magnesium. So, we can say that there exists a diagonal relationship between magnesium and lithium.