164 Neutron
Enter James Chadwick. Born in 1891 in England, Chadwick was another of the crop of
physicists who learned their atomic physics under Rutherford. By the mid-1920s Chadwick
was obsessed with the search for Rutherford’s uncharged proton-electron.
In 1928 Chadwick began to use beryllium for his experiments. Beryllium was a small,
simple atom with an atomic mass of 9. He bombarded beryllium with alpha particles from
polonium (a radioactive element) and hoped that some beryllium atoms would be struck by
alpha particles and burst apart into two new alpha particles (each with a mass of 4).
If that happened, these two alpha particles would carry all of the electrical charge of
the original beryllium nucleus, but not all of its mass. One atomic unit of mass (the mass of a
proton) would be left over from beryllium’s original mass of 9. But that last proton-sized
particle from the breakup