Answer: neutron
Notes: Mass of proton is 1.672621777(74) × 10-27 kg. An electron has a mass 9.10938291(40) ×10-31 kg that is approximately 1/1836 that of the proton. The mass of neutron is slightly larger than that of a proton. The mass of the hydrogen nucleus is 1.7 x 10-27 kg. The heaviest of these particles is the neutron. Nucleus in hydrogen atom is devoid of neutron. It has proton only.
