Final answer:
Calcium, with an atomic number of 20, has 2 electrons in its outermost shell. Upon forming a Ca²+ cation, it loses those electrons and takes on the electron configuration of argon, with 18 electrons remaining.
Step-by-step explanation:
The element calcium (Ca), which has an atomic number of 20, has 2 electrons in its outermost shell. The electronic configuration of calcium is [Ar]4s², indicating that the outermost shell has two valence electrons. Calcium belongs to group 2 in the periodic table, which is the alkaline earth metals group. These elements typically have two electrons in their outer shell.
When calcium forms a cation by losing those two valence electrons, it achieves the electron configuration of the noble gas preceding it, which in this case is argon (Ar). As a result, the calcium cation, represented as Ca²+, will have 18 electrons, reflecting the electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁶, which is isoelectronic with argon.