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 nucl