An atom has three fundamental partcles viz electrons, protons and neutrons.
1. ELECTRON: The lightest fundamental particle which is negatively charged[-1.6×10^(-19) C]. It revolves around the nucleus by following definite orbits called shells.
2. PROTON : Proton is a positively charged fundamental particle which is inside the nucleus.
3. NEUTRON : The neutral fundamental particle which with protons forms the nucleus of the atom.The masses of neutron and Proton determine the mass of the atom