proton

An elementary particle found in the nuclei of atoms. It belongs to the class of particles called baryons.

unit positive electric charge
spin = ½
mass of 1.671 621 58 × 10^-27 kilogram, with a standard uncertainty of 0.000 000 13 × 10^-27 kg.¹

A proton is not point-like, but has a “diameter.” A proton is made of 3 quarks (2 up and 1 down), and the proton’s diameter is the average distance between the quarks.

Do protons decay?

The question of whether protons spontaneously change into another particle (a positron, probably) remains one of the great unanswered questions of physics. Experiments have shown that, if protons do decay, on the average they must have a half life of more than 1.6 × 10²⁵ years.²

1. 1998 CODATA value.

2. Evans and Steinberg.
Science, volume 197, page 989. (1977)

resources

The Particle Data Group offers authoritative data, an interesting chart and links to related sites: http://pdg.lbl.gov