Where is most of the mass of an atom?
Almost all of the mass of an atom is in the nucleus. The nucleus is made up of protons and neutrons, with hydrogen-1 as the lone exception because its nucleus has only a proton in it. In other atoms, protons and neutrons are fused together to form that nucleus. The electrons are out in their electron cloud. And we mention the electron because it has only about 1/1836th the mass of a proton. See how it works? A fistful of electrons weigh almost nothing compared to a single proton. The neutron is slightly heavier than a proton, so when neutrons and protons are gathered in any atomic nucleus, it contains most all of the mass of that atom.