What is the chemical equation for calcium + phosphorus ---> calcium phosphide?

Calcium has a charge of $C {a}^{+ 2}$.Phosphorous has a charge of ${P}^{-} 3$. To bond these two ions together, their charges must be equal and opposite. Therefore we need 3 $C {a}^{+ 2}$ ions to balance with 2 ${P}^{-} 3$ in order to balance their charges at 6 for the molecule $C {a}_{3} {P}_{2}$.
$3 C a + 2 P \to C {a}_{3} {P}_{2}$