ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

The majority of an atom's mass is attributed to the nucleus, which contains protons and neutrons. Rutherford's pioneering experiments in the early 20th century led to the discovery of the atomic nucleus. He proposed a model of the atom in which most of the atom's mass is concentrated in this central region, surrounded by a cloud of electrons. Because protons and neutrons are significantly more massive than electrons, the contributions of the electrons to the overall mass of an atom are negligible. In essence, while protons and neutrons each carry a substantial amount of mass, it is their presence in the nucleus that constitutes the bulk of an atom's overall mass. This understanding was fundamental in advancing the modern atomic model.