A Neutron is a subatomic particles which are found inside an atom, the nucleus. They also have a neutral charge unlike protons which has a positive charge or an electron which has a negative charge. So, if an atom has equal numbers of electrons and protons ,e.g. carbon 6 and 6, the charges cancel each other out and the atom has a neutral charge. You could add a thousand neutrons into the mix and the charge would not change. But in doing so it would severely change the mass and the radioactive properties of atoms.
Like protons, neutrons are too small to see, even with an electron microscope, but we know they must be there because that's the only way we can explain how atoms behave. All neutrons, everywhere in the universe, are exactly the same, and pretty much all of them are inside atoms or in Neutron stars. Also like protons, neutrons have a size of 10-13 cm and a mass of 1.67×10−27 .
Neutrons are actually made of even smaller invisible particles, called quarks. Like protons, each neutron is made of three quarks, but neutrons are made of two down quarks and one up quark. A strong nuclear force sticks the quarks together. Most of the mass of a neutron comes from this strong nuclear force, rather than from the quarks.
Who discovered the neutron?
The British experimental physicist James Chadwick, who spent more than a decade searching, discovered the neutron in 1932. Of the three fundamental particles that make up atoms, the electron, the proton and the neutron, the neutron was the last to be discovered. It's lack of a charge made it more elusive than its companions. Because it was uncharged there would be no electrical repulsion of the neutron as it passed through matter, so it would be much more penetrating than the proton. This would make the neutron difficult to detect. The existence of a "neutral element" within the atom was suggested by Ernest Rutherford in 1920 as Rutherford known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus.
How to discover a neutron?
In 1930 the German physicists Bothe and Becker bombarded the light metal beryllium with alpha particles, and noticed that a very penetrating radiation was emitted. This radiation was non-ionising, and they assumed it