The fundamental achievement of physical science is the atomic model of matter. That model is simplicity itself. All matter is composed of atoms, which themselves form aggregates called molecules. An atom contains a positive nucleus very much smaller than the full atom. A nucleus with atomic mass A contains Z protons and A − Z neutrons. The neutral atom has, as well, Z electrons, each with a mass only 1/1836 that of a proton. The chemical properties of the atom are determined by Z; atoms with equal Z but differing A have the same chemistry and are known as isotopes.

This school-level description did not exist at all in 1895. Atoms were the creation of chemists and were still distrusted by many physicists. Electrons, protons, and neutrons were yet to be discovered. Atomic spectra were well studied, but presented a bewildering catalog of lines connected, at best, by empirical rules like the Balmer formula for the hydrogen atom. Cathode rays had been studied, but many regarded them as uncharged, electromagnetic waves. Chemists had determined the atomic weights of the known elements and Mendeleev had produced the periodic table, but the concept of atomic number had not yet been developed.

The discovery of X-rays byW. C. Röntgen in 1895 began the revolution that was to produce atomic physics. Röntgen found that cathode-ray tubes generate penetrating, invisible rays that can be observed with fluorescent screens or photographic film.