Magnesium is most likely to form a cation, specifically the Mg2+ ion. This means that it loses two electrons to achieve a stable electronic configuration, resembling that of the nearest noble gas, neon.
When magnesium loses these electrons, it becomes positively charged, hence it is classified as a cation. Cations are formed by elements that have low electronegativity and tend to lose electrons easily. In the case of magnesium, being an alkaline earth metal, it readily gives up its two outermost electrons to fulfill the octet rule, resulting in a stable and positively charged ion.