When metals react, they lose electrons. This is because metals typically have fewer electrons in their outer shell, which are loosely held and easier to lose. By losing electrons, metals form positive ions (cations) and achieve a stable electron configuration usually resembling that of the nearest noble gas. This process is called oxidation. In contrast, nonmetals gain electrons to form negative ions (anions). The transfer of electrons from metals to nonmetals leads to the formation of ionic bonds.