A fundamental unit of matter having a positive charge and a mass number of 1, equivalent to 1.67 × 10−24 gram. Its mass is 1837 times that of the negatively charged electron, but is almost identical with that of the uncharged neutron. Protons are constituents of all atomic nuclei, their number in each nucleus being the atomic number of the element. An atom of normal hydrogen contains one proton and one electron. A proton is identical with a hydrogen ion (H+).