Question

What is the formula unit of sodium nitride?

Answer 1

Hint: Sodium nitride is an ionic compound which is actually considered to be very unstable alkali metal nitride. It can be produced by combining the atomic beams of metal i.e. sodium and nitrogen that is deposited onto a substrate (i.e. low-temperature sapphire). It is employed as a food preservative.

Complete step by step answer:
As the name suggests, the given compound sodium nitride comprises a metal sodium and a non-metal nitrogen. We know that any compound containing a metal and a non-metal is considered to be an ionic compound. In ionic compounds, we have to take into account the charges of each of the elements present in the compound. Now let us identify the formula for the given compound i.e. sodium nitride.
From the periodic table, it is clear that sodium is an alkali metal having a symbol $$Na$$ and it belongs to group 1 thus, it possesses +1 cationic charge while nitrogen is a non-metal having a symbol $$N$$and it possesses -3 anionic charge. It will look like $N{a}^{1+}{N}^{3-}$. As sodium nitride is neutral, the net charge on the compound should be zero. So in the present case, $(+1)+(-3)=-2\ne 0$. Thus, this can’t be the formula for sodium nitride as one to one ratio of the charges makes the ionic formula. Now, we can change the subscripts in order to balance the charge. We will change the subscript of both metal and non-metal following the criss-cross method to neutralise the charge i.e.$(N{a}^{1+}{)}_3(N^{3-}{)}_1$. Now the charges are balanced as $(3\times (+1))+(1\times (-3))=0$.
Hence, the formula for sodium nitride is $N{a}_{3}N$.

Note: While writing the chemical formula of an ionic compound, never write the subscript '1'. And it should be noted that we can also have one or two polyatomic ions like $$N{H}_{4}N{O}_3$$ in the same compound. In this case, we have to find and write their names as identified from the Common Ion Table.