Which halogen has atomic number 17?

The halogen with atomic number 17 is chlorine. Its chemical symbol is Cl, and it occupies group 17 of the periodic table, the group known as the halogens. Chlorine is the second-lightest halogen, between fluorine and bromine.

At room temperature, elemental chlorine is a yellow-green gas with a sharp odor. It is highly reactive because each chlorine atom has seven valence electrons and tends to gain one more electron, forming the chloride ion. Chlorine is therefore a strong oxidizing agent, though fluorine is more reactive.

A common mix-up is confusing chlorine gas with chloride. Chloride is the negatively charged ion found in compounds such as sodium chloride, ordinary table salt. Chlorine compounds are widely used in disinfecting drinking water and swimming pools, while chlorine chemistry also plays a role in bleaching and manufacturing. Chlorine’s atomic number identifies its 17 protons; isotopes may have different numbers of neutrons.