Final answer:
Calcium and sulfur form an ionic compound with the formula CaS, each contributing a 2+ and 2- charge, respectively, resulting in a neutral compound.
Step-by-step explanation:
When calcium (Ca) and sulfur (S) combine to form an ionic compound, the calcium atom, which has a charge of 2+ (Ca2+), will typically lose two electrons, and the sulfur atom, which will gain two electrons, forms a sulfide ion with a 2- charge (S2-). The resulting ions are Ca2+ and S2-. When these two ions combine to form an ionic compound, they do so in a ratio that balances the total positive and negative charges. Hence, for each Ca2+ ion, one S2- ion is required, leading to a 1:1 ratio. Therefore, the correct formula is CaS, which means the answer is option A) CaS.