Neutron
A neutron is a particle with no electric charge found in the nucleus of an atom; it adds mass and helps hold the nucleus together.
Definition
A neutron is a particle in the nucleus with no electric charge and a mass about equal to a proton ($1.675 \times 10^{-27}\text{ kg}$, or $1.00866\text{ amu}$, very slightly heavier). Neutrons add to the mass number but not the atomic number, and atoms of the same element with different neutron counts are isotopes. Made of one up quark and two down quarks, neutrons stabilize nuclei by adding strong-force attraction without adding electrostatic repulsion; a free neutron is unstable and beta-decays into a proton, an electron, and an antineutrino with a mean lifetime of about $15$ minutes.
Example
A helium nucleus has $2$ protons and $2$ neutrons; the neutrons add mass but no charge. Carbon-12 has $6$ neutrons and carbon-14 has $8$, so both are carbon but carbon-14 is heavier and radioactive. To count neutrons, subtract the atomic number from the mass number: chlorine-37 has $37 - 17 = 20$. Light stable nuclei have about equal protons and neutrons, but heavy ones need extra neutrons, so lead-208 has $82$ protons and $126$ neutrons.
Key Insight
Neutrons are neutral, so they neither attract nor repel electrons and leave no ionization trail, which is why Chadwick did not identify them until 1932, decades after the electron and proton. Nuclei with too many or too few neutrons decay radioactively, and free neutrons are what sustain the chain reaction inside a nuclear reactor.