For a chlorine atom to become a chloride ion, it must gain an electron. The chloride ion has a negative charge and is written as Cl-. Chlorine gains an electron because it has seven valence electrons, and to be a full octet, it gains an electron.
A chloride ion is called an anion. Its electron configuration is 1s2 2s2 2p6 3s2 3p6. The electron configuration of chlorine is 1s2 2s2 2p6 3s2 3p5.
Chlorine, which is a halogen, has 17 protons,17 electrons and 18 neutrons. It atomic weight is 35.453 grams per mole. It is a greenish-yellow gas that is toxic and can irritate the respiratory tract, skin and eyes.