The central core of an atom that contains most of its mass. The nuclei of atoms are positively charged and consist of one or more protons and neutrons. The number of protons in a nucleus defines the atomic number of the element. The simplest nucleus is that of the lightest isotope of hydrogen, consisting of one proton only. All other nuclei also contain one or more neutrons, which contribute to the mass but not the charge of the nucleus. The most massive nucleus that occurs naturally on Earth is ²³⁸U, containing 92 protons and 146 neutrons.