Science, Tech, Math › Science Proton Definition - Chemistry Glossary What Is a Proton? Share Flipboard Email Print A proton is a positively-charged particle. Science Photo Library - MEHAU KULYK, Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated January 07, 2018 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.