Final answer:
A calcium ion (Ca²⁺) has 20 protons and 18 electrons, which results from a neutral calcium atom losing two electrons. This ion carries a 2+ charge and has an electronic structure similar to argon.
Step-by-step explanation:
A calcium ion has 20 protons and 18 electrons. This is because a neutral calcium atom, which has an atomic number of 20, loses two electrons to become a Ca²⁺ ion. Consequently, this calcium ion carries a 2+ charge, indicating it has two more protons than electrons. The process of forming a calcium ion involves the loss of two valence electrons, resulting in a cation that is isoelectronic with the noble gas argon.
For instance, when calcium donates two electrons to chlorine atoms during the formation of calcium chloride (CaCl2), calcium becomes a Ca²⁺ ion with a full octet resembling that of argon, fulfilling the octet rule. This transfer of electrons highlights the ionization energy concept, which indicates the energy required to remove electrons from an atom, becoming greater as more electrons are removed.