Here's how to determine the ionic formula for gallium and sulfur:

* Identify the charges:

* Gallium (Ga) is in Group 13, so it typically forms a +3 ion (Ga³⁺).

* Sulfur (S) is in Group 16, so it typically forms a -2 ion (S²⁻).

* Balance the charges: To achieve a neutral compound, you need two Ga³⁺ ions (total +6 charge) and three S²⁻ ions (total -6 charge).

* Write the formula: The ionic formula is Ga₂S₃.