Neutrons
A neutron is slightly heavier than a proton. A neutron has a mass of 1.00867u and a proton has a mass of 1.00728u. The difference, which is the mass of the electron, is 0.00055u.
A free neutron disintegrates into a proton and an electron. There is a release of energy but the net electric charge remains as zero.
Protons, neutrons and electrons