An atom is a complex arrangement of electrons that orbit about a positively charged nucleus containing (except for hydrogen) protons and neutrons. Most of the mass of an atom is concentrated in the dense nucleus.
The proton is one of two types of particles found in the nucleus. The existence of a positively charged particle, a proton, in the nucleus was proved by Sir Ernest Rutherford in 1919. A proton has a mass of 1.67 x 10 kg.
