Deuterium

Urey's approach was to collect a large volume of liquid hydrogen and then to allow that liquid to evaporate very slowly. His hypothesis was that the lighter and more abundant protium isotope would evaporate more quickly than the heavier hydrogen-2 isotope. The volume of liquid hydrogen remaining after evaporation was nearly complete, then, would be relatively rich in the heavier isotope.

In the actual experiment, Urey allowed 4.2 qt (4 l) of liquid hydrogen to evaporate until only 0.034 oz (1 ml) remained. He then submitted that sample to analysis by spectroscopy. In spectroscopic analysis, energy is added to a sample. Atoms in the sample are excited and their electrons are raised to higher energy levels. After a moment at these higher energy levels, the electrons return to their ground state, giving off their excess energy in the form of light. The bands of light emitted in this process are characteristics for each specific kind of atom.

By analyzing the spectral pattern obtained from his 0.034 oz (1 ml) sample of liquid hydrogen, Urey was able to identify a type of atom that had never before been detected, the heavy isotope of hydrogen. The new isotope was soon assigned the name deuterium. For his discovery of the isotope, Urey was awarded the 1934 Nobel Prize in chemistry.