Metals tend to lose electrons from their outermost shell during chemical reactions to achieve stability. For example, lithium has three electrons in total, with one in its outer shell. It becomes stable by losing that one electron.
Similarly, aluminium has 13 electrons in total, with three in its outer shell. It achieves stability by losing these three electrons.
Metals can be classified as either reactive or unreactive, based on their tendency to undergo chemical reactions. Potassium is an example of a highly reactive metal, whereas platinum is an example of a relatively unreactive metal.
The reactivity of metals can be organised into a reactivity series, with the most reactive metals at the top and the least reactive at the bottom.