Final answer:
A Calcium (Ca) atom will have 20 electrons when it is neutral, but it will lose two electrons when it forms a cation, resulting in a calcium ion with 18 electrons and a 2+ charge. charge, and an electron configuration of 1s²2s²2p63s²3p6. The Ca²+ ion is therefore isoelectronic with the noble gas Ar.
Step-by-step explanation:
When calcium (Z = 20) gains two electrons the resulting ion will have 18 electrons and 20 protons and therefore a charge of +2 (there are two more positive protons than negative electrons), it is a cation. The symbol for a calcium ion is Ca²+. The charge is represented as a superscript on the right side of the element symbol, e.g. H+ is hydrogen without any electron, i.e., 1 proton, 0 neutrons, and 0 electrons. 20Ca²+ is calcium with two fewer electrons than protons, i.e., 20 protons, 20 neutrons, and 18 electrons.
When forming a cation, an atom of a main group element tends to lose all of its valence electrons, thus assuming the electronic structure of the noble gas that precedes it in the periodic table. For groups 1 (the alkali metals) and 2 (the alkaline earth metals), the group numbers are equal to the numbers of valence shell electrons and, consequently, to the charges of the cations formed from atoms electrons and a ground state electron configuration of 1s²2s²2p63s²3pº4s². When a Ca atom loses both of its valence electrons, the result is a cation with 18 electrons, a 2+ charge, and an electron configuration of 1s²2s²2p63s²3p6. The Ca²+ ion is therefore isoelectronic with the noble gas Ar.