To determine the formula of the compound that forms between cesium (Cs) and sulfur (S) using Lewis theory, we start by considering the valence electrons of each element.
Cesium is an alkali metal found in Group 1 of the periodic table and has one valence electron. Sulfur, on the other hand, is a chalcogen located in Group 16 and has six valence electrons.
According to Lewis theory, atoms tend to bond in such a way that allows them to achieve a full valence shell, typically following the octet rule. Cesium, with its one valence electron, can easily lose that electron to achieve a stable electron configuration similar to that of the noble gas xenon. On the other hand, sulfur needs two additional electrons to fill its valence shell and reach the stable configuration of argon.
When cesium and sulfur react, cesium will donate its one valence electron to sulfur. However, since sulfur requires two electrons to achieve a full valence shell, two cesium atoms are needed to donate one electron each to one sulfur atom. This transfer of electrons results in the formation of a compound where two cesium cations (Cs+) bond with one sulfur anion (S2-).
Thus, the formula for the compound that forms between cesium and sulfur is Cs2S. This reflects the stoichiometry required to satisfy the charge balance, leading to a stable ionic compound.