Hint: Neutrons are the subatomic particles which along with protons make up the nucleus of an atom. They do not have any charge on them and are neutral entities. Other than neutrons, protons and electrons are also found in an atom.
Complete answer: -Neutrons were discovered by James Chadwick in the year $1932$. -Rutherford predicted the presence of neutrons by his alpha scattering experiment. -In this experiment he bombarded a thin gold sheet with alpha particles and discovered that most of the mass of an atom is concentrated in the centre, and named it as the nucleus. -James Chadwick bombarded a piece of beryllium kept in a vacuum chamber by alpha radiations -When those rays struck beryllium particles, neutral rays were emitted. -He then struck various targets with this radiation. -When the radiation struck paraffin wax, protons were emitted from hydrogen atoms, which recoiled with a high velocity. -Many scientists thought that this highly penetrating radiation consists of high energy photons. -But Chadwick concluded that photons having no mass can not strike out a proton from the atom whose mass is more than that of photons. -Since neutrons have no charge on them, they can penetrate deep into the atom to strike the proton. -He thus proposed the existence of a neutron having mass approximately the same as that of a proton.
Note: The experiment including the study of unknown radiation emitted by beryllium atoms when struck by alpha particles was first conducted by Frédéric and Irène Joliot-Curie. Chadwick wasn’t satisfied by their conclusion of the experiment and conducted a similar experiment to discover neutrons.