Chlorine (Cl). Diagram of the electron configuration of an atom of chlorine (atomic number: 17). The nucleus of protons and neutrons is at centre. 17 electrons (red) bind to the nucleus, successively occupying available electron shells (rings). The stability of an element's outer electrons determines its chemical and physical properties. Vital to life in ionic form, chlorine is a halogen in group 17, period 3, and the p-block of the periodic table. In elemental form it is a yellow-green, reactive, toxic gas (Cl2) that liquefies at minus 34 degrees Celsius.