Hydrogen (symbol, H; atomic mass, 1.00794) has three isotopes. Two are stable in natural environments and are denoted as 11H and 12H (deuterium or D). The third isotope 13H (tritium or T) is unstable and decays to 3He with a half-life of 12.32 years. Being hydrogen one of the water-forming elements, its isotopic ratio D/H has been used for identifying the origin of water within the Solar System. Particularly, the chondritic D/H ratio measured in meteorites and Antarctic micrometeorites (143–170 × 10−6) is closer to the value of the bulk Earth (149–153 × 10−6) and the present-day ocean (155.7 × 10−6) than the comets (298–324 × 10−6). This is the isotopic evidence often used for claiming that water on Earth has been delivered by chondritic planetesimals rather than comets.