An electrically charged atom is called an ion. Ions are formed when an atom either loses or gains an electron. Atoms tend to move toward a more stable electron configuration, which happens when an atom possesses an outer energy level filled with electrons.
The most stable electron arrangement is to have eight electrons in the outermost energy level. This concept is called the octet rule. To achieve this configuration, atoms lose, gain or share electrons. Atoms that have few electrons in their outermost energy levels usually lose these electrons to other atoms to become positively charged ions, or cations. Those atoms that have six or more electrons in their outer energy levels tend to gain more electrons to become more stable. These atoms become negative ions, or cations.