A proton is a subatomic particle with a positive electric charge. It sits in the nucleus of an atom, alongside neutrons. The number of protons determines the element. Hydrogen has one. Helium has two. Carbon has six. Gold has seventy-nine. Protons are made of three quarks: two up quarks and one down quark. The strong nuclear force binds those quarks together. Protons are stable in most nuclei, but a free proton is stable on its own. It does not decay, at least not on any timescale we have observed.

Protons were discovered by Ernest Rutherford in 1917, though he did not name them until later. The word proton comes from the Greek meaning first, because protons were thought to be the fundamental building blocks of matter. That view changed with the discovery of quarks, but the name stuck. Protons are about 1,836 times more massive than electrons. The mass of a proton is about 1.672 × 10⁻²⁷ kilograms. The number of protons in a nucleus determines its atomic number, which determines the element. Changing the number of protons changes the element. Adding a proton to a nitrogen nucleus turns it into oxygen.

Proton facts

• Charge. Positive, +1.6 × 10⁻¹⁹ C.
• Mass. About 1,836 times the electron mass.
• Composition. Two up quarks, one down quark.
• Discovery. Ernest Rutherford, 1917.
• Role. Determines the element.

A proton is not the same as a neutron. A proton has positive charge. A neutron has no charge.