Subatomic Particles

Dalton's atomic theory successfully explained the laws of chemical combination, but it treated the atom as an indivisible, hard sphere and could not explain everyday observations such as glass or ebonite becoming electrically charged when rubbed with silk or fur. The discovery of subatomic particles in the late 19th and early 20th centuries broke this picture apart. Three subatomic particles matter to chemistry: the electron, the proton and the neutron. The proton and the neutron sit together inside a tiny central nucleus and are jointly called nucleons, while the electron occupies the much larger extranuclear space surrounding that nucleus. Each of these three particles carries a distinct combination of charge and mass: the electron is negatively charged and extremely light (about 1/1837th the mass of a proton, so its mass is often treated as negligible, i.e. $\approx 0\,u$), the proton carries a single positive charge and a mass of about 1 u, and the neutron is electrically neutral with a mass also close to 1 u. Because the proton and neutron have almost equal mass and the electron's mass is negligible by comparison, essentially all of an atom's mass is concentrated in its nucleus, even though the nucleus occupies only a minute fraction of the atom's total volume.