Atomic Models

The discovery of the proton resulted in another mystery. The mass of the hydrogen atom was known to be larger than the mass of a proton and an electron added together. Scientists searched for the source of the missing mass by assuming that another particle must exist in the nucleus of the atom that also contributes to the mass of the atom. This particle must be neutral in charge, since the positively charged protons cancel out the charge of the negatively charged electrons, and the atom as a whole is neutral. Because it is electrically neutral, the detection of this missing particle was problematic. Approximately 30 years after the electron was discovered, this third particle was found.

Irene Joliot-Curie (1897–1956) performed experiments in which she bombarded a beryllium sample with alpha particles. These experiments resulted in a new beam with a higher penetrating power than the beam of alpha particles. In 1932, the British scientist James Chadwick (1891–1974) discovered that this new beam was composed of particles of approximately the same mass as protons. In addition, magnetic or electric fields could not deflect this beam. Chadwick concluded that the beam must be made up of neutral particles of approximately the same size as protons, which he called neutrons. Neutrons, together with protons, make up the nucleus of the atom and contribute to the majority of its mass. Electrons are located in the empty space surrounding the atom, which makes up most of its volume. The mass of the electron is insignificant when compared to the mass of the protons and neutrons.