A negatively charged atom is called an anion. An anion forms when an atom gains electrons from another atom. Atoms are generally neutral, but they take on a charge when they gain or lose electrons.
The formation of an ion follows the octet rule, which states that an atom is most stable when its outermost shell, or energy level, contains eight electrons. With eight electrons, an atom's outer electron is completely filled.
Highly electronegative atoms tend to form anions. These atoms have almost a full complement of electrons in their outermost energy levels; therefore, they tend to gain, rather than lose, electrons to form stable ions. Elements belonging to the far right side of the periodic table, the nonmetals, typically form anions; elements on the left side, the metals, form cations, or positively charged ions.