Neutron is an electrically neutral particle with a mass similar to that of proton. Neutrons belong to a heavy subclass and consist of two bottom quarks and a top quark. Most nuclei are composed of neutrons and protons (except that an isotope of hydrogen atoms is composed of one proton). Outside the nucleus, neutrons are inherently unstable, with a half-life of 15 minutes. Neutron decay releases an electron and an antineutrino into protons. The same decay process exists in some nuclei. Neutrons and protons in the nucleus can be transformed into each other by absorbing and releasing π mesons.