Neutron is a subatomic particle similar to proton in mass and composition. Just like proton, it is made of three quarks, hence the similar, yet slightly higher mass, approximately one atomic mass unit or 1.675 x 10^-27 kg. However, unlike proton, neutron has no electric charge. Together with protons, neutrons make atomic nuclei and are also called nucleons.
Neutrons are found in all atomic nuclei except that of the most common hydrogen isotope which has just one proton. Other hydrogen isotopes can have one or two, rarely more neutrons, which means they are heavier the more neutrons they have, but their electric charge remains the same and chemically they are the same element. Generally, many elements have isotopes with more or less neutrons than they have protons and they thus have different masses.
Neutron is fermionic particle, which means it has ½ spin. Being made of quarks, it is also a hadron and more specifically, a baryon, because it is made of three quarks. This internal composition is the reason why neutrons don’t have electric charge. The three quarks a neutron is made of are two down quarks with -⅓ of the elementary charge each and one up quark with +⅔ of the elementary charge and their sum electric charge is zero. Proton has two up quarks and one down quarks with total electric charge of +1.
Since a down quark is heavier than an up quark, neutrons have slightly higher mass than protons. Inside of a neutron, quarks are bound together by the strong force. Just like in a proton, mass of three quarks makes up only about 1% of neutron’s mass. The remainder of the mass comes from quarks’ kinetic energy and the energy of gluon field that holds quarks together.
In atomic nucleus, a neutron is bound to a proton by the secondary effects of the strong force inside them. Bound to protons, neutrons are mostly stable. When free, neutron is unstable and by the means of weak force, a single neutron decays into a proton, electron and antineutrino.
It was theorised in 1920 by Ernest Rutherford, but finally discovered in 1932 by James Chadwick, who three years later received the Nobel Prize for this discovery.
Neutrons have a wide application, most notably in developing nuclear reactors and nuclear weapons. When atomic nucleus captures a free neutron, it becomes heavier and enters excited state, which leads to radioactivity. We utilised this by bombarding heavy nuclei of elements like uranium with neutrons, after which uranium fractures into lighter elements. This is called nuclear fission and in the process, large amounts of energy are released. Depending on a process, this energy can be used for destructive purposes or as a source of electric power.