Answer
Ca's two valence electrons complete the octet in the two Cl atoms' valence shells.
Work Step by Step
Ca has two s subshell electrons in its outer shell. The Cl atoms are each missing just one electron to complete an outer shell. Each of the two s electrons of Ca can fit into a “spare electron” state of a Cl atom, which makes an ionic bond.
The three atoms now have filled outer shells. This is similar to the case of NaCl, diagrammed in Figures 29–4 and 29–5.
