Nitrogen is a chemical element with the symbol N and atomic number 7. It is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. Nitrogen is a colorless, odorless, tasteless gas that makes up 78% of Earth's atmosphere and is a constituent of all living matter. It is estimated to be the seventh most abundant element in the Milky Way and the Solar System. Nitrogen is remarkably inert and does not readily react with other substances. It also participates in the nitrogen cycle, a biogeochemical cycle in which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems.