Negatively charged ions are called anions. Atoms that are high on the electronegativity scale, such as fluorine and oxygen, become anions by gaining electrons. Highly electronegative atoms strongly attract electrons, thus attaining eight electrons in their outer energy levels and creating a more stable atom.
Atoms in their natural states are neutral, meaning they have the same numbers of protons and electrons. Protons have a positive charge, and electrons have a negative charge. In a neutral atom, the number of negative charges in the energy levels balances the number of positive charges in the nucleus.
The octet rule governs the activity of atoms. Atoms can become more stable by achieving full outer energy levels like the elements called the noble gases. Atoms that have close to eight electrons in their outer energy levels tend to gain electrons rather than lose them. These atoms become anions, which usually consist of the nonmetallic elements of the periodic table.