Final answer:
A calcium atom has two valence electrons, which it can lose to form a Ca2+ cation isoelectronic with argon, particularly while forming compounds like calcium chloride by transferring electrons to chlorine atoms.
Step-by-step explanation:
A calcium atom has two valence electrons. Calcium, being a group 2 element, has its valence electrons in the 4s orbital. As per the octet rule, atoms seek to have eight electrons in their valence shell. Calcium can achieve a stable electronic configuration by losing its two valence electrons, resulting in the formation of a Ca2+ cation, which is isoelectronic with the noble gas argon (Ar).
Based on the question, in the formation of calcium chloride, Ca atoms lose these two valence electrons to two Cl atoms, each needing one electron to complete its octet. This transfer of electrons is the key to forming ionic compounds like CaCl2, where calcium acts as a cation with a +2 charge.