Neutron
Year of Discovery: 1932
What Is It? A subatomic particle located in the nucleus of an atom with the
mass of a proton but no electrical charge.
Who Discovered It? James Chadwick
Why Is This One of the 100 Greatest?
The discovery of neutrons has been hailed as a major landmark of twentieth-century
science. First, this discovery completed our understanding of the structure of atoms. Second, because they have no electrical charge, neutrons have been by far the most useful particles for creating nuclear collisions and reactions and for exploring the structure and reaction
of atoms. Neutrons were used by Ernest Lawrence at UC Berkeley to discover a dozen new
elements. Neutrons were essential to the creation of nuclear fission and to the atomic bomb.
How Was It Discovered?
Since the discovery that a subatomic world existed (in 1901), only the two electrically
charged particles—proton and electron—had been discovered. Scientists assumed that
these two particles made up the whole mass of every atom.
But there was a problem. If atoms were made up of protons and electrons, spin didn’t
add up correctly. The idea that each subatomic particle possessed a “spin” was discovered
in 1925 by George Uhlenbeck and Samuel Goudsmit in Germany. For example, a nitrogen
atom has an atomic mass of 14 (a proton has a mass of 1) and its nucleus has a positive electrical charge of +7 (each proton has a charge of +1); to balance this positive charge, seven
electrons (charge of -1 each) orbit around the nucleus. But somehow, seven additional electrons had to exist inside the nucleus to cancel out the positive electrical charge of the other
seven protons.
Thus, 21 particles (14 protons and 7 electrons) should reside in each nitrogen nucleus,
each with a spin of either +½ or -½. Because 21 is an odd number of particles, no matter how
they combined, the total spin of each nitrogen nucleus would have to have a ½ in it. But the
measured spin of a nitrogen nucleus was always a whole integer. No half. Something was
wrong.
Ernest Rutherford proposed that a proton-electron must exist and that a nitrogen nucleus has seven protons and seven proton-electrons (for 14 particles—an even number—
and the correct spin total). But it was only theory. He had no idea of how to detect a proton-electron since the only known way to detect a particle was to detect its electrical charge.
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