* Calcium (Ca) is a metal in group 2 of the periodic table, meaning it has two valence electrons. It tends to *lose* these electrons to achieve a stable, noble gas configuration.

* Sulfur (S) is a nonmetal in group 16, meaning it has six valence electrons. It tends to *gain* two electrons to achieve a stable, noble gas configuration.

When calcium and sulfur react, calcium loses its two valence electrons, becoming a positively charged ion (Ca²⁺). Sulfur gains these two electrons, becoming a negatively charged ion (S²⁻). These oppositely charged ions attract each other electrostatically, forming an ionic bond.

The resulting compound, calcium sulfide, is a solid with a high melting point, typical of ionic compounds.