Proton

The proton is a subatomic particle with a positive electric charge and a mass roughly equal to that of a neutron. It is one of the fundamental constituents of atomic nuclei and a key determinant of an element’s identity.

Protons are baryons composed of three valence quarks—two up quarks and one down quark—held together by gluons through the strong interaction. They have spin 1/2 and a magnetic moment; the electric charge is +e, and their size is about 0.84 to 0.87 femtometres in radius.

In nuclei, protons are bound with neutrons by the strong force. The number of protons in a nucleus, called the atomic number Z, defines the element (for example, hydrogen has Z = 1, carbon Z = 6). The overall charge of a neutral atom equals Z times the elementary charge.

Protons participate in electromagnetic, strong, and weak interactions. The strong force binds protons and neutrons in the nucleus, while electromagnetic repulsion among protons is offset by the strong interaction. Weak interactions can transform protons into neutrons in certain processes, such as beta decay.

Historically, protons were identified as the positively charged component of atomic nuclei in the early 20th century; the term proton was introduced by Ernest Rutherford. In modern science, protons are produced and studied in particle accelerators, while practical applications include proton therapy for cancer and the study of hydrogen nuclei in materials and biology.