In 1917, Rutherford discovered that firing alpha particles at nitrogen gas atoms results in the production of hydrogen nuclei. He concluded that these positively charged hydrogen nuclei must have originated from the nuclei of the nitrogen atoms. This marked the discovery of the proton.
Meanwhile, there was speculation that there was something else in the nucleus. For example, the helium nucleus was known to have the charge of 2 protons but the mass of roughly 4 protons. Rutherford believed that the “extra” mass belongs to a yet-to-be-discovered uncharged particle in the nucleus. This particle was eventually named the neutron when Chadwick proved its existence in 1932.
So, after almost 20 years of intensive research, the components of an atom have finally been confirmed: electrons, protons and neutrons. Since then, we have made very precise measurements of their mass and charge.
Note that
- The unified atomic mass unit u is by definition the mass of
the mass of a neutral carbon-12 atom. Since one mole of carbon-12 atoms has a mass of exactly 12 g,
- The electron has only 1/1833 times the mass of a proton, but carries the exact same magnitude of charge.
- The neutron is approximately the same mass as the proton, but carries zero charge.
- More precisely, the neutron is a tiny wee bit more massive than the proton. It kind of makes sense later when you learn that a neutron can actually “split” to become a proton plus an electron.
xmPhysics 