Proton Definition - Chemistry Glossary

The primary parts of an atom are protons, neutrons, and electrons. Take a closer look at what a proton is and where it's found.

Proton Definition

A proton is a component of an atomic nucleus with a mass defined as 1 and a charge of +1. A proton is indicated by either the symbol p or p⁺. The atomic number of an element is the number of protons an atom of that element contains. Because both protons and neutrons are found in the atomic nucleus, they are collectively known as nucleons. Protons, like neutrons, are hadrons, composed of three quarks (2 up quarks and 1 down quark).

Word Origin

The word "proton" is Greek for "first." Ernest Rutherford first used the term in 1920 to describe the nucleus of hydrogen. The existence of the proton had been theorized in 1815 by William Prout.

Examples of Protons

The nucleus of a hydrogen atom or the H⁺ ion is an example of a proton. Regardless of the isotope, each atom of hydrogen has 1 proton; each helium atom contains 2 protons; each lithium atom contains 3 protons and so on.

Proton Properties

• Because opposite charges attract each other, protons and electrons are attracted. Like charges repel each other, so two protons exert repulsion on each other.
• Protons are stable particles that do not decay into other particles. Free protons are common, often formed when sufficient energy is available to separate protons from electrons.
• Free protons are found in plasma. About 90 percent of cosmic rays consist of protons.
• The radioactive decay of free neutrons (which are unstable) may produce protons, electrons, and antineutrinos.