A proton is approximately 2,000 times heavier than an electron. Protons and electrons, along with neutrons, are the principal sub-atomic particles that make up an atom.
Protons and electrons have equal but opposite charges. In most cases, atoms have an equal number of protons and electrons. However, because protons are so much heavier than electrons, the electrons do not make a significant contribution to the mass of an atom. The masses of atoms are expressed in relative mass units called atomic mass units or a.m.u. Protons have a mass of 1.007277 a.m.u., and electrons have a mass of 0.00054858 a.m.u.