Write down the chemical formula of:
Magnesium nitride

Hint: For writing the chemical formula, one needs to know the chemical symbol of the element, its valency and any charge if present on an ion. Then, by cross multiplying the valencies, we can find the chemical formula of the compound.

Complete answer:
> For writing the chemical symbols, first, we need to know the chemical symbol of the elements involved. Further, we need to know the valency of the element. For example - in the above question, we have the Mg for magnesium and ‘N’ for nitride.
> The Magnesium has two valence electrons and its valency is +2. It can be written as - $M{g^{2 + }}$
In case, there is an ion, we need to know the charge on that ion. Such as in this question, we have nitride ions. It has a charge of -3 on it. It can be written as - ${N^{3 - }}$.
> To derive the chemical formula, now we need to cross multiply their valencies. So, we get $M{g_3}{N_2}$.

Thus, the chemical formula of magnesium nitride is $M{g_3}{N_2}$.

Note: It must be noted that we all know that nitride ions have -3 charge. It is the nitrogen atom when it has accepted three electrons. Normally, nitrogen exists in the diatomic form in the gaseous state. The nitride ion is the monatomic form. The valency of an element is the maximum number of electrons that can be donated or accepted by an atom whichever is easy to do till it achieves noble gas configuration.