Deuterium

"Deuterium (symbol D or 2H, also known as heavy hydrogen) is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in 6,420 of hydrogen (~156.25 ppm on an atom basis). Deuterium accounts for approximately 0.0156 percent (or on a mass basis: 0.0312 percent) of all the naturally occurring hydrogen in the oceans, while the most common isotope (hydrogen-1 or protium) accounts for more than 99.98 percent. The abundance of deuterium changes slightly from one kind of natural water to another (see VSMOW)."

"The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the far more common hydrogen isotope, protium, has no neutron in the nucleus. The deuterium isotope's name is formed from the Greek deuteros meaning "second", to denote the two particles composing the nucleus.^[1] Deuterium was discovered and named in 1931 by Harold Urey, earning him a Nobel Prize in 1934. This followed the discovery of the neutron in 1932, which made the nuclear structure of deuterium obvious. Soon after deuterium's discovery, Urey and others produced samples of "heavy water" in which the deuterium has been highly concentrated with respect to the protium."