Final answer:
Magnesium needs to lose two valence electrons to achieve a stable electron configuration, resulting in a Mg2+ ion. The 1s, 2s, and 2p subshells are completely filled, while higher subshells beyond 3s are unfilled.
Step-by-step explanation:
Magnesium, or Mg, is an alkaline earth metal with an atomic number of 12. It needs to lose two electrons to attain a full outer shell, which will give it a noble gas configuration, similar to neon (Ne). The process of losing two electrons results in a magnesium ion with a +2 charge, represented as Mg2+. Magnesium has a valence shell configuration of [Ne]3s2, meaning it has two valence electrons in the 3s subshell. This subshell is completely filled, but losing these two electrons leads to a stable, filled neon-like electron configuration. When discussing how many subshells are completely filled with electrons for Mg, we see that Mg has the electron configuration of [Ne]3s2, indicating that the 1s, 2s, and 2p subshells are completely filled, since it shares the same inner electron configuration as neon. However, after the 3s subshell, all higher subshells are unfilled. Therefore, Mg needs to lose electrons from its outermost subshell, which is the 3s subshell, to become stable.