Polyatomic Reaction Charges

Question

When a polyatomic ion reacts with another atom to create an ionic compound, often, we know the charge of the polyatomic ion. But how can we infer the charge of the atom it is reacting with?

For example, the question states, 'What is the correct formula for Calcium Phosphate?'

I know that the charge of the polyatomic ion named Phosphate is 3-. But does that mean the charge of Calcium, in this case, must be 3+ in order for it to react with Phosphate? If not, how can we figure out the charge of the Calcium ion in this ionic bond?

Answer 1

When a Calcium atom loses two electrons, it attains the stable electronic configuration of the noble gas Argon. Thus, Calcium cannot have a $\ce{+3}$ charge in any ionic compound. So, the ions forming your salt are $\ce{Ca^2+}$ and $\ce{PO4^3-}$. We need to balance the charges to obtain a neutral salt. We can see that the least common multiple of the charges (2 and 3) is 6. A total charge of $\ce{+6}$ can be obtained by taking three Calcium cations, and this can be balanced by taking 2 Phosphate anions.

Calcium Phosphate is $\ce{Ca3(PO4)2}$