Urey’s original chart
Urey’s chart as of 2012
Other stable isotopes
Unstable but long lived
Unstable: 5He and 8Be
(Left) Isotope chart constructed by Harold C. Urey in 1931 in his search for deuterium. The labels dem-
onstrate the conception of nuclear structure at the time: The nucleus was thought to consist exclusively
of protons and tightly bound “nuclear-electrons.” The neutron was unknown. Filled circles indicate
known isotopes; open ones show those that might be expected to be found by following an obvious
trend. Urey saw this as a “road map” to the likely existence of deuterium. (Right) Today’s road map,
showing the stable isotopes known to Urey (black); stable isotopes discovered since—just 2H and 3He
(blue); and long-lived but unstable isotopes of practical importance, including the neutron (red). Two
isotopes indicated in the chart on the left have since been found to be unstable, 5He and 8Be.
t dawn on Thanksgiving Day, 1931, no one in the world had an accurate idea of the
nature of the atomic nucleus. That day, at Columbia University in New York, Harold C.
Urey and George M. Murphy measured the optical emission spectra of samples of hydro-
gen gas received by railway express shipment from the National Bureau of Standards in Washington,
D.C., U.S.A. Urey arrived home late for Thanksgiving dinner, but with the news that he had discovered
the mass 2 isotope of hydrogen. He later remarked, “I thought maybe my discovery might have the prac-
tical value of, say, neon in neon signs. My colleagues felt I was exaggerating [its] importance.”
As it happens, the discovery by optical spectroscopy of that isotope, which Urey and his collabora-
tors subsequently named “deuterium,” transformed our understanding of nuclear structure. It made
possible the first thermonuclear explosion 21 years later, and, just this past December, it provided
perhaps the first direct glimpse of primordial gas created in the Big Bang.
A history of isotope chemistry
In the early 20th century, chemists were puzzled by the existence of isotopes: atoms of the same
chemical element with different weights. These atoms had been discovered in 1912 by Soddy in his
study of the decay of uranium to radon. He realized that there could be versions of an element whose
masses were different, even though their chemical properties were the same. He named this concept
an isotope, which is Latin for “same place.” In other words, atoms of differing weight could occupy
the same place in the periodic table as the original element. In 1913, J.J. Thomson succeeded in separating isotopes of neon by passing a beam of neon ions through a magnetic field, which deflects an
ion in proportion to the ratio of its electric charge and mass.
Aston’s construction of a mass spectrograph in 1919 made possible the discovery of many other
isotopes of stable elements. By Thanksgiving Day 1931, almost all of the stable isotopes of light atoms
that are known today had been found, mostly via mass spectroscopy.
Everything changed within a few months. Deuterium, the heavy stable isotope of hydrogen, was
discovered Thanksgiving afternoon in the optical spectrum of the hydrogen atom. The neutron was discovered in February 1932. Shortly thereafter, Werner Heisenberg’s suggestion that neutrons and protons
were alternative quantum states of the same particle deepened physicists’ understanding of the structure
of the nucleus, and the electrolysis of water proved to be an efficient means for producing deuterium.
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