Answer:
CaS
Step-by-step explanation:
Calcium is a metal from Group 2, so it has 2 valence electrons. Sulfur is a non-metal from Group 16, so it has 6 valence electrons. Metals and non-metals form ionic bonds, in which metals lose electrons and non-metals gain electrons. In both cases, they follow the octet rule, trying to have the electron configuration of the closest noble gas, which has its valence shell complete with 8 electrons.
Calcium has 20 electrons and loses 2 electrons to have the electron configuration of Ar and form Ca²⁺.
S has 16 electrons and gains 2 electrons to have the electron configuration of Ar and form S²⁻.
In the formula unit, one atom of Ca²⁺ bonds to one atom of S²⁻ to maintain electroneutrality and form CaS.