The smallest particle of the atom is the electron, with a mass of 1/1840 that of the proton or neutron. It's actual mass in grams is 9.11E-28, and the actual mass in grams of the proton and neutron is 1.67E-24. Most of the atom's mass is concentrated in the nucleus, which accounts for a very small part of the volume of an atom. The nearly massless electrons are outside the nucleus.
In addition to having a much smaller mass and being located in a different part of the atom, the electron has a different charge from the other two particles. The electron is negative, while the proton has a positive charge. The neutron is neutral as it has no net electrical charge.