The philosophers who first hypothesized the existence of “atoms” had in mind the smallest particles of matter that could preserve identifiable chemical properties – building blocks that could be assembled into familiar substances. Little more than this definition – tiny masses that carry kinetic energy and undergo collisions – led to the fruitful quantitative explanation of vapor properties in the kinetic theory of gases, and to such devices as mass spectrometers and ion accelerators.

Atoms and molecules. As became clear during the early twentieth century, these “chemical atoms’ from which materials are constructed have internal structure that endows them with their chemical attributes: one or more positively charged nuclei, each a few femtometers (1 fm = 10−13 cm = 10−15 m) in diameter, surrounded by one or more much lighter negatively charged electrons whose motion fills a volume of at least a few cubic angstroms (1 Å= 10−8 cm = 10 nm) in diameter. Nowadays we distinguish between particles having multiple nuclei (molecules) and those with a single nucleus (atoms); when the positive and negative charges are unbalanced these are ions (positive or negative). The simplest atom, hydrogen, has a single electron; the most complex atoms have more than a hundred electrons. Although I will often refer to “atoms”, usually the discussion applies equally well to molecules or to any other structure whose constituents exhibit distinct quantum properties, as manifested by discrete energies.

Nuclei. The nuclei of atoms are, in turn, composed of protons and neutrons.