Protons are found inside the nucleus of an atom, which is the central and most massive part of the atom. The nucleus contains protons and neutrons, which are collectively referred to as nucleons. Protons carry a positive electric charge, while neutrons are electrically neutral. The number of protons in the nucleus determines the atomic number of an element and is what distinguishes one element from another. For instance, all atoms with one proton are hydrogen atoms, those with two protons are helium atoms, and so on.