Why do zinc, cadmium, and mercury commonly form ions with a 2+ charge?

Zinc, cadmium, and mercury commonly form ions with a 2+ charge because they can lose the two electrons in their outermost shell to achieve a stable configuration. By losing these two outer electrons, the elements expose the next innermost shell, which possesses a stable configuration of 18 electrons.1

For example, zinc has an electron configuration of [Ar]3$d^{10}$4$s^2$, and cadmium has an electron configuration of [Kr]4$d^{10}$5$s^2$. In both cases, the loss of the two $s$-orbital electrons results in a stable, filled $d$-subshell.231

While these elements can theoretically lose more electrons under conditions of intense heat or powerful electric or magnetic fields, ordinary chemical reactions generally do not provide sufficient energy to remove electrons beyond this stable 18-electron configuration. Consequently, the +2 oxidation state is the most common for these metals.12