On the periodic table, which element is the halogen in period 3?

On the periodic table, chlorine is the halogen in period 3.

Chlorine has atomic number 17 and belongs to group 17, the halogens. Its outer electron arrangement makes it highly reactive, and it commonly forms the chloride ion by gaining one electron. At room temperature, elemental chlorine is a greenish-yellow gas.

Chlorine is widely used to disinfect drinking water and swimming pools because reactive chlorine compounds can destroy many microorganisms. It is also important in manufacturing plastics, solvents, pharmaceuticals, and other chemicals.

Elemental chlorine should not be confused with chloride. Chloride is the negatively charged ion found in table salt, sodium chloride, and in body fluids. Fluorine and bromine are also halogens, but fluorine is in period 2 and bromine is in period 4.