A picture of a proton. The 'u' stands for an up quark, and the 'd' stands for a down quark.

A proton is part of an atom. They are found in the nucleus of an atom along with neutrons. The periodic table groups atoms according to how many protons they have. A single atom of hydrogen (the lightest kind of atom) is made up of an electron moving around a proton. Most of the mass of this atom is in the proton, which is almost 2000 times heavier than the electron. Protons and neutrons make up the nucleus of every other kind of atom. In any one element, the number of protons is always the same. An atom's atomic number is equal to the number of protons in that atom.