The proposition that matter consists of small particles originated in antiquity, but it took the work of Antoine Lavoisier to establish the law of conservation of mass in chemical reactions as the basis for establishing the atomic theory. John Dalton expanded upon Lavoisier's work by establishing Dalton's law of definite proportions that described how a few atoms of one element could combine with a few atoms of another to form molecules. Dimitry Mendeleyev developed a scheme to arrange the elements in an orderly fashion called the periodic table.

Electrons are particles with a mass of 9.1 x 10^-31 kg and a charge of -1.6 x 10^-19C. The natural radiation called beta particles (b) actually consists of electrons. A second form of natural radiation, alpha particles (a), consists of ionized helium atoms. Alpha particles have a positive charge that is double that of the electron. X-rays are electromagnetic radiation of very short wavelength, much shorter than that of visible light. This is why x-rays are used for medical imaging.

The Balmer formula accounts for the wavelengths of the light emitted by hydrogen when the gas is placed in a tube with a high voltage across the electrodes at the ends of the tube. Planck proposed that the light emitted by a blackbody came in discrete chunks or quanta called photons whose energy depended upon the frequency.

Niels Bohr proposed a model for the atom that had electrons in orbits around a nucleus. Light was emitted when an electron jumped from one stable orbit to another, and the frequency of the light emitted could be calculated from the energy differences between the two orbits. His model gave predictions in good agreement with the observed hydrogen spectrum.

DeBroglie suggested that just as light could be considered as simultaneously having wave properties and particle properties, matter could be considered to have both wave and particle properties.

Max Planck suggested that in some cases light seemed to behave as though it were made up of discrete bundles of energy called photons. He also proposed that the energy of the light was a product of the constant now named after him, h, and the frequency of the light, f, in the equation E = h f. This proposal represented the attribution of particle-like properties to light. DeBroglie completed the cycle by suggesting that particles exhibit wave-like properties with the wavelength of the particle calculated as Planck's constant divided by the momentum, l h / p.

The Heisenberg uncertainty principle states that the position and momentum of a particle cannot both be known simultaneously with high precision. The product of the uncertainty in position and momentum must be greater than or equal to Planck's constant, h = 6.626 x 10^-34 J s.