The additional neutron makes a deuterium atom roughly twice as heavy as a protium atom. Water enriched in molecules that include deuterium instead of protium is called heavy water. There is a large difference in scattering length between protium (-0.374) and deuterium (0.667). Urey created the names "protium", "deuterium", and "tritium" in an article published in 1934. The ordinary isotope of hydrogen, with no neutrons, is sometimes called protium. Naturally occurring water is almost completely composed of the neutron-less hydrogen isotope protium. Typical hydrogen is called hydrogen-1 or protium and has no neutrons. Protium is stable and makes up 99.985% of naturally occurring hydrogen atoms.