1 atom of calcium is needed to form an ionic bond with sulfur.
If calcium has two valence electrons and it needs to denote these two electrons to achieve a noble gas electronic configuration, and sulfur has six electrons, it needs two extra electrons to achieve a noble gas electronic configuration. Thus a ionic bond is formed between sulfur and calcium, in which 1 atom of calcium donate its 2 electrons to 1 atom of sulfur and formation of bond takes place.
