Deuterium

The possible existence of an isotope of hydrogen with atomic mass of two was suspected as early as the late 1910s after Frederick Soddy had developed the concept of isotopes. Such an isotope was of particular interest to chemists. Since the hydrogen atom is the simplest of all atoms—consisting of a single proton and a single electron—it is the model for most atomic theories. An atom just slightly more complex—one that contains a single neutron—also could potentially contribute valuable information to existing atomic theories.

Among those who sought for the heavy isotope of hydrogen was Harold Urey, at the time professor of chemistry at Columbia University. Urey began his work with the realization that any isotope of hydrogen other than hydrogen-1 (also known as protium) must exist in only minute quantities. The evidence for that fact is that the atomic weight of hydrogen is only slightly more than 1.000. The fraction of any isotopes with mass greater than that value must, therefore, be very small. Urey designed an experiment, therefore, that would allow him to detect the presence of heavy hydrogen in very small concentrations.