When a metal reacts, it typically loses electrons to form positive ions known as cations. This happens because metals have a tendency to lose electrons in order to achieve a stable electron configuration. For example, sodium loses one electron to become Na+, and magnesium loses two electrons to become Mg2+. These positive ions (cations) are formed by the loss of electrons from the metal atoms during chemical reactions, especially when reacting with non- metals which gain those electrons and form negative ions (anions).