Q. Composition of the nucleus

Since the time of Rutherford, other scientists have learned a significant amount about the fundamental properties about the nucleus of an atom. In particular, they were able to discover that the nucleus was composed of fundamental particles called protons and neutron.

The Proton

In an experiment where he used a modified cathode-ray tube, Thompson realized that there were rays traveling in a direction opposite to that of the negative cathode-rays made up of electrons. Thompson showed that these rays were composed of positively-charged particles that have the same amount of electrical charge, but of opposite sign, as electrons. These particles were eventually given the name of protons.

The Neutron

Scientists were confused as they realized that the total mass of the atom was more than just the sum of the masses of the protons and electrons. They suspected that the atom contained a third type of subatomic particle that contributed to the atomic mass.

However, as this particle did not have an electrical charge, and therefore was neutral, it was more difficult to detect that the electron and proton. In 1932, an English scientist named James Chadwick showed that an atom contains a third kind of subatomic particle which he called a neutron. A neutron has about the same mass as a proton and is also found in the nucleus of the atom. However, unlike the proton, the neutron does not have an electric charge.