Evidence for Neutrons Now we've got electrons and a positive nucleus...how do we know the nucleus has both protons and neutrons? Rutherford, based on experimental evidence, concluded that the positive parts of all atoms were made of hydrogen nuclei, which he gave the name protons. There were still some things that didn't make sense, though... Like what? Well, take helium, for instance. An average helium nucleus has is four times as heavy as a proton, which seemed to imply that it was made of four protons. But then it ought to have four times the electric charge of a proton, whereas in fact it has only two times this charge. All the heavier nuclei showed a similar mismatch between mass and charge. Even worse, different isotopes of some elements had been discovered--atoms of the same element with different masses. In 1920, Rutherford guessed that there had to be a new kind of particle in the nucleus, about as heavy as a proton but with no electric charge, which he called the neutron. And then Rutherford actually found some neutrons, right? No, but his assistant James Chadwick did, in 1932. Neutrons appeared in yet another experiment involving alpha particles. This time, they had been shot at beryllium; the beryllium atoms then emitted a weird kind of neutral radiation. Some people suggested that this consisted of high-energy photons, like gamma rays, but Chadwick showed that that couldn't be. The unknown radiation could knock protons out of other atoms; that meant it had to be a fairly heavy particle, not a massless photon. Using the velocity of the knocked-out protons and the laws of conservation of energy and momentum, Chadwick calculated the mass of the unknown particle. It was just a little heavier than the proton. He had no doubt that these were Rutherford's neutrons.

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