Here's how to understand the formula:

* Magnesium (Mg): Magnesium is a group 2 element, forming a +2 cation (Mg²⁺).

* Hydrogen Phosphite (HPO₃⁻): This is a polyatomic ion with a -1 charge.

* Balancing Charges: To balance the charges, we need two hydrogen phosphite ions (2 x -1 = -2) to counter the +2 charge of the magnesium ion.

Therefore, the final formula is Mg(HPO₃)₂.