Now, a nucleus is composed of two types of particles called protons and neutrons. Now, protons and neutrons have approximately the same weight. A neutron is a tiny bit heavier than protons, but for all purposes we can approximate that these guys have the same exact mass. Electrons, however, have a very small mass, much smaller than that of protons or neutrons. In fact, it's 1800 times smaller than a proton or a neutron. Now, if we look at this table and we look at their masses, a proton has one AMU, a neutron has one AMU.