Deuterium, or H or sometimes D, is another stable isotope of hydrogen. The isotope has one proton and one neutron. It makes up about 0.0026 – 0.0184% of all atoms of hydrogen on Earth. Deuterium is less in hydrogen gas on Earth and is more in sea water (0.015% or 150 ppm). Deuterium is not radioactive, and it will not harm living things. Deuterium can also form water molecules. Water that has deuterium instead of protium is named heavy water.