Chlorine belongs to the halogen family of elements, which is group seven on the periodic table. Halogens have seven electrons in their outermost energy levels, which makes them extremely reactive. In compounds, halogens usually gain electrons and become negative ions.
Chlorine, with seven electrons in its outermost energy level, tries to gain an electron to complete the energy level and attain a more stable configuration. Because it is so reactive, chlorine is never found in its elemental form in nature. Like the other halogens, chlorine forms bonds easily with metals, especially those in the alkali metals family.
Chlorine is the third-most electronegative element on the periodic table. Electronegativity is a measure of how much an atom attracts electrons. All elements in the halogen group are extremely electronegative.