The question of the building blocks of matter has preoccupied scientists for centuries. After John Dalton put forward his atomic hypothesis at the beginning of the 19th century, it was only a few years before another scientist, William Prout, hypothesised the structure of atoms in 1815.

Hydrogen was known to be the lightest element. Based on the atomic weights determined at the time, which were apparently multiples of the atomic weight of hydrogen, he surmised that all elements are made up of hydrogen. All heavier elements were a composition of several atoms of hydrogen.

Prout's hypothesis only held up for a few years, as more precise determinations of the atomic masses revealed that these were not integers for every element.

Around 100 years later, in 1919, Ernest Rutherford continued his research into the interaction of α-particles with matter. In an experiment, he bombarded gaseous nitrogen with α-particles and was able to observe the formation of hydrogen nuclei using the same detection method as in the famous scattering experiments.

Rutherford surmised that the hydrogen nuclei were knocked out of the nuclei of the nitrogen atoms by the α particles, causing the nitrogen nucleus to transform. With these experiments, Rutherford had found a building block of atomic nuclei - the nucleus of a hydrogen atom - and Prout's hypothesis proved to be not entirely wrong.

This particle, with a mass of 1 and a single positive charge, was called a proton (from the ancient Greek 'the first').