In the early part of the twentieth century, the experiments carried out by Ernest Rutherford and his colleagues led to the idea that at the centre of an atom there is a tiny but massive nucleus. At the same time that the quantum theory was being developed and that scientists were attempting to understand the structure of the atom and its electrons, investigations into the nucleus itself had also begun. An important question to physicists was whether the nucleus had a structure, and what that structure might be. In fact, it has so far turned out that the nucleus is a complicated entity, and even today, it is not fully understood. However, by the early 1930s, a model of the nucleus had been developed that is still useful. According to this model, a nucleus is considered as an aggregate of two types of particles: protons and neutrons. A proton is the nucleus of the simplest atom which is hydrogen. The neutron, whose existence was ascertained only in 1932 by the English physicist James Chadwick, is electrically neutral as its name implies. These two constituents of a nucleus, neutrons and protons, are referred to collectively as “nucleons.”