When a calcium atom loses two electrons, it forms a calcium ion with a positive charge. A neutral calcium atom has 20 electrons, corresponding to its atomic number. By losing two electrons, the number of electrons in the calcium ion becomes:
20 (original electrons) – 2 (lost electrons) = 18 electrons
Thus, the calcium ion has 18 electrons. This change in electron count results in a net charge of +2 for the ion, as it now has more protons (20) than electrons (18).