Atoms that have gained or lost electrons are called ions. If an atom gains electrons, it is called an anion; if it loses electrons, it becomes a cation. Through the gain or loss of electrons, atoms are no longer neutral.
When atoms gain electrons, they become negatively charged. Atoms losing electrons become positively charged. In a neutral atom, the number of electrons orbiting the nucleus equals the number of protons in the nucleus of the atom. Protons are the positive particles in an atom, while electrons are the negative ones. A neutral atom has the same number of positive as negative charges. With the positive and negative charges canceling each other out, a neutral atom has no net charge.
With the gain or loss of electrons, the balance of protons and electrons is shifted. Either the atom contains more protons than electrons or more electrons than protons, resulting in the atom gaining a net charge.