How do you predict the correct formula for the combination reaction between a nonmetal and a group a metal?

Refer to the Periodic Table.

Explanation:

To predict the correct formula for the combination reaction between a nonmetal and a metal group, you can simply refer to the Periodic Table and check what formula does it have for any particular element.

The Periodic Table has both the metal and non-metal group with their atomic masses and other characteristics. Looks for the formula of each element in the compound and add the number of molecules according to the chemical equation.