Neutrons are fundamental particles found in the nucleus of atoms (with the exception of the most common isotope of hydrogen). They are electrically neutral, possessing zero charge. Their mass is very close to that of a proton, slightly greater but often considered almost equal for simplicity. Protons and neutrons collectively form the atomic nucleus, which is located at the center of the atom. Electrons, which are negatively charged particles, revolve around the atomic nucleus in specific energy levels or orbitals.