In physics, the proton is a subatomic particle with a positive
fundamental electric charge of 1.6 × 10-19 coulomb, a mass of
938 MeV (1.6726231 × 10-27 kg, or about 1800 times that of an
electron) and a half-life of about 1033 years. The nucleus of
the most common isotope of the hydrogen atom, H, is a single
proton. The nuclei of other atoms are composed of neutrons and
protons held together by the strong nuclear force. The number of
protons in the nucleus determines the chemical properties of the
atom and what chemical element it is. Because the
electromagnetic force is many [[order of magnitude|orders of
magnitude]] stronger than the gravitational force, the charge on
the proton must be equal to the charge on the electron,
otherwise the net repulsion of having an excess of positive or
negative charge (depending on which charge was numerically
greater - atoms would not be electrically neutral) would cause a
noticeable expansion effect on the universe, and indeed any
gravitationally aggregated matter (planets