Chadwick's neutron chamber
containing parallel disks of radioactive polonium and beryllium. Radiation is emitted from an aluminium window at the chamber's end

1. What is inside the nucleus?

By 1910 the atom was thought to consist of a massive nucleus orbited by electrons, but measurements of atomic mass indicated that all nuclei must contain integer numbers of some other particle. What were these particles inside the nucleus?

One of these particles was the proton. The proton was discovered during investigations of positive rays, and can be produced by ionising hydrogen. Hydrogen is the lightest type of atom, consisting of a single proton and a single electron. Ionisation separates the electron from the atom, so only the proton remains.

If more massive nuclei contained only protons their charge would be much higher than measurements suggested. With the exception of hydrogen all atoms have a higher mass number than charge number. Rutherford thought that the nucleus consisted of protons and 'neutral doublets' formed from closely bound protons and electrons. This could explain both the mass and the charge that had been measured for different nuclei.

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