Final answer:
Calcium and chlorine both achieve the stable electron configuration of argon ([Ar]) when they form the ionic compound calcium chloride.
Step-by-step explanation:
When calcium (Ca) and chlorine (Cl) react to form the ionic compound calcium chloride (CaCl2), they undergo changes in their electron configurations to acquire the stable electron configuration of a noble gas. Calcium, being a Group 2 element with an electron configuration of [Ar]4s², loses two valence electrons and becomes a Ca²+ cation with an electron configuration of [Ar]. Chlorine, on the other hand, gains an electron to fill its outer p orbital and becomes a Cl⁻ anion with an electron configuration of [Ne]3s²3p⁵. After the electron transfer, both ions obtain the electron configuration of argon, which is 1s²2s²2p⁶3s²3p⁶.