How does oxygen form a negative ion when reacting with magnesium?

When magnesium reacts with oxygen, the formation of a negative ion occurs through the process of electron transfer. Magnesium, which typically exhibits a +2 oxidation state, loses two electrons from its outer shell.1

Oxygen possesses high electronegativity and electron affinity, meaning it has a strong tendency to attract electrons to fill its two half-filled outer orbitals. When magnesium donates its two electrons to oxygen, these orbitals are filled, resulting in the creation of the oxide ion, $O^{2-}$, which carries a -2 charge. This complete transfer of electrons is a defining characteristic of the reaction between these elements, leading to the formation of magnesium oxide.21