A chloride ion has 18 electrons total. The chloride ion has an added electron to complete the outermost electron shell, or valence shell, of the atom.
Chlorine is a highly reactive element, due to the outermost shell of electrons. Electrons fill rings, known as shells, outside the atom. The first shell, closest to the atom, holds two electrons. Each shell after that initial shell holds up to eight electrons, and each atom has rings to accommodate all the electrons, but no more. Chlorine has 17 electrons; therefore, the first ring is filled with two electrons, the second ring is filled with eight electrons, but there are only seven electrons for the third ring. This makes chlorine's valence shell incomplete, and the atom attracts "loose" electrons to this shell to complete it, so the atom can be stable. Cloride is a chlorine atom, which has found that last electron and added it to the outermost valence shell to become stable. This added electron gives the chloride ion a negative charge and is thus considered an anion.