Final answer:
The correct ionic formula when Ga3+ and S2- ions react is Ga2S3. This results in a neutral compound where the charges of the ions are balanced, with two gallium ions and three sulfide ions.
Step-by-step explanation:
The correct ionic formula when Ga3+ (gallium ion with a +3 charge) and S2- (sulfide ion with a -2 charge) react is Ga2S3. This is because in order to achieve a neutral compound, the total positive charge must balance the total negative charge. Gallium has a +3 charge and sulfide has a -2 charge, so two Gallium ions will give a total charge of +6, and three sulfide ions will give a total charge of -6. When combined, they cancel each other out resulting in a neutral compound.