In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939.573 MeV/c² or 1.008 664 915 (78) u (1.6749 × 10−27 kg, slightly more than a proton). Its spin is ½. Its antiparticle is called the antineutron. The neutron, along with the proton, is a nucleon.
The nucleus of all atoms (except the lightest isotope of hydrogen, which has only a single proton) consists of protons and neutrons. The number of neutrons determines the isotope of an element. For example, the carbon-12 isotope has 6 protons and 6 neutrons, while the carbon-14 isotope has 6 protons and 8 neutrons. Isotopes are atoms of the same element that have the same atomic number but different masses due to a different number of neutrons.
A neutron consists of two down quarks and one up quark. Since it has three quarks, it is classified it as a baryon.