A proton is a basic particle located in the nucleus of an atom. The number of protons in the nucleus governs the chemical properties of that element. The positive electrical charge is equal to the negative charge of an electron and the mass is similar to that of a neutron (see also baryon). The proton belongs to the group of hadrons and is composed out of three quarks.
A hydrogen (H1) nucleus is build of one proton.
Proton beam therapy is the preferred treatment for cancers near the eye and the spinal cord.

[Radioactive Isotope] Radioisotopes are atoms with an unstable nucleus, which try to reach a more stable state by emitting radiation. Through one or more steps down its decay chain a more stable number of protons and neutrons is reached. Approximately 3,800 natural and artificial (fission, generator) radioisotopes exist. Radioisotopes are commonly used in science, industry, and nuclear medicine.

See also Isotope, Decay and Decay Chain.