Chemical Formula of Aluminium Nitride is

Aluminium belongs to group 13 and forms a trivalent cation ${\text{Al}}^{3+}$. Nitrogen belongs to group 15 and forms a trivalent nitride anion ${\text{N}}^{3-}$. When they combine, the valencies balance each other out in a $1:1$ ratio, resulting in the chemical formula $\text{AlN}$.