Proton

A proton is a tiny, positively charged particle that exists in the nucleus of an atom. The nucleus of each atom consists of protons and neutrons (uncharged particles), except in ordinary hydrogen, which has only a single proton.

The number of protons in the nucleus determines the type of element. This number is called the atomic number. For example, an atom with one proton is hydrogen, two protons is helium, and six protons is carbon. Variation in the number of neutrons does not change the element type, but creates different isotopes.

Basic properties

A proton carries one unit of positive charge, equal in magnitude but opposite in sign to the negative charge of an electron. Its mass is approximately 1,836 times greater than that of an electron. For this reason, most of the mass of an atom is contained in the nucleus.

Internal structure

A proton is not indivisible. It is understood to consist of three fundamental quarks (two up and one down), held together by gluons, which mediate the strong force. Thus the proton is classified as a hadron, specifically a baryon, and is not elementary like an electron.

Stability and decay

A free proton is considered highly stable; experiments have found no evidence that it decays. Some theories of grand unification predict an extremely small possibility of decay, but this remains unconfirmed. When a proton is in the nucleus, it can transform into a neutron through a process called beta-plus decay.

Applications and history

The discovery of the proton came from scattering experiments in the early twentieth century, which showed that atoms contain a small, dense nucleus. Today, protons are accelerated in high-energy machines to study the structure of matter, and are also used in cancer treatment in a method known as proton therapy.