The small, central, positively charged central core of an atom. Except for the nucleus of ordinary (light) hydrogen, which has a single proton, all atomic nuclei contain both protons and neutrons. The number of protons determines the total positive charge, or atomic number, which in turn determines the chemical element that a given atom represents. That is to say, all atoms of a given chemical element have the same number of protons in their nuclei. The total number of neutrons and protons is called the mass number.