Deuterium, also called heavy hydrogen, has an atomic mass of 2.014 u. Deuterium is one of two stable isotopes of hydrogen. In contrast, hydrogen has an atomic mass of 1.008 u.
Common hydrogen has a nucleus containing a single proton and no neutron. Deuterium has a nucleus containing a single proton as well as a single neutron. According to the Encyclopedia Britannica, deuterium is an atomic species found in natural hydrogen compounds to the extent of 0.014 to 0.015 percent. Deuterium was discovered in 1931 by American physical chemist Harold C. Urey. He named the isotope from the Greek word "duteros" meaning "second" to denote the two particles composing the nucleus.