Inside the Nucleus

We now know that the nucleus is comprised of protons and neutrons, but Rutherford's experiment showed only that there was a small positively charged nucleus. It was Rutherford himself in 1919 who discovered that the nucleus contains positive charge in discrete units which he called protons. He was experimenting again with alpha particles, but this time in nitrogen gas. He found that the alphas knocked out long-range particles, which were just like hydrogen nuclei, so he concluded that the hydrogen nucleus "which is liberated formed a constituent part of the nitrogen nucleus". Rutherford named this constituent the proton after 'protos', the Greek for first.

The neutron, being an uncharged particle, evaded discovery for longer. Rutherford surmised that the nucleus must contain a neutral particle in order to account for the mass of the atom, which was found to be about twice that due to the protons needed to provide the positive charge. However, he thought the neutral object was a combination of an electron and a proton. The main evidence for the neutron came from an experiment by James Chadwick at Cambridge in 1932. Other researchers had found that firing alpha-particles at beryllium caused the emission of a neutral radiation that could penetrate 200 mm of lead. Also, when a block of paraffin wax was placed in front of this radiation, protons (nuclei of hydrogen in the paraffin) were emitted from the wax. Chadwick studied the neutral rays by sending them into other materials, such as nitrogen gas, and found that these nuclei would also recoil. By comparing the recoils in different materials he was able to determine that the neutral radiation was in fact a beam of neutral particles - neutrons - whose mass he measured to be just greater than that of the proton.

James Chadwick

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