A particle in the nucleus is a subatomic particle that is found inside the nucleus of an atom. The three most common particles in the nucleus are neutrons, protons, and mesons. Neutrons are neutral particles with a mass of 1.675 x 10^-27 kg and a charge of 0. Protons have a mass of 1.673 x 10^-27 kg and a charge of 1. Mesons are particles with a mass between that of the electron and the proton and a charge of either 1 or -1.