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A proton is a tiny particle, smaller than an atom. Protons are too small to see, even with an electron microscope, but we know they must be there because that's the only way we can explain how atoms behave.

Protons make the nucleus, or center, of an atom. The simplest atoms - hydrogen atoms - have a nucleus made of just one proton. But most atoms have more than one proton, and at least one neutron to go with each proton.

Protons are actually made of even smaller invisible particles, called quarks. Each proton has three quarks, two up quarks and one down quark. A strong nuclear force sticks the quarks together. Most of the mass of a proton comes from this strong nuclear force, rather than from the quarks.

Because a proton has two up quarks and only one down quark, it has a positive charge, like the positive end of a magnet. Two things that have positive charges push away from each other, so the protons in an atom are always pushing away from each other. They need the neutrons to pull them together. The neutrons help to stick the protons together, using the strong nuclear force. The heaviest atom that exists in nature is uranium, with 92 protons.