From speculation to experiment
In the 5th century BC the Greek thinkers Leucippus and Democritus argued that cutting matter again and again must end in indivisible grains, atomos, an idea passed on in the Roman poet Lucretius's De Rerum Natura. Long overshadowed by Aristotle's four elements, atomism revived in 1808 when John Dalton proposed that each element has atoms of a characteristic weight and that compounds combine them in whole-number ratios. Even so, late-19th-century scientists such as Ernst Mach treated atoms as a convenient fiction. In 1905 Albert Einstein explained the jittering of pollen grains in water — Brownian motion — as molecular collisions, and in 1909 Jean Perrin confirmed it experimentally, settling the debate over whether atoms exist.
How atomic models changed
In 1897 J. J. Thomson showed that cathode rays are negatively charged particles far lighter than atoms — electrons — and proposed the "plum pudding" model. In 1911 Ernest Rutherford saw that some alpha particles fired at gold foil bounced back and concluded that almost all of an atom's mass sits in a tiny central nucleus. In 1913 Niels Bohr explained the hydrogen spectrum with electrons that occupy only fixed energy orbits, and after Erwin Schrödinger's 1926 wave equation electrons came to be pictured as probability clouds (orbitals) rather than tracks — the atom of quantum mechanics. James Chadwick's discovery of the neutron in 1932 showed the nucleus holds protons and neutrons, which are themselves made of quarks.
How small, and how empty
An atom is roughly a ten-millionth of a millimetre across: if an apple were blown up to the size of Earth, its atoms would be about the size of apples. The nucleus is some 100,000 times smaller still — in an atom the size of a football stadium, it would be a pea at the centre. Atoms are therefore almost entirely empty, and solidity comes from electrical repulsion between electrons and quantum effects. The number of protons, the atomic number, defines the element, and arranging elements by it gives the periodic table. Electrons jumping between energy levels absorb and emit light, and heavy nuclei can split and release huge energy in nuclear fission. The scanning tunnelling microscope built by Gerd Binnig and Heinrich Rohrer in 1981 first made individual atoms visible.
FAQ
- Can we actually see atoms?
- Not with ordinary light microscopes, since atoms are much smaller than light's wavelength, but scanning tunnelling, atomic force and modern electron microscopes can image arrangements of atoms.
- Can atoms be split?
- Despite the name, atoms consist of electrons, protons and neutrons, and protons and neutrons are made of quarks. Chemical reactions, however, never split atoms.
Sources reviewed: 2026-10-26
References
- Lucretius. De Rerum Natura (M. F. Smith, Trans., 2001). Indianapolis: Hackett.
- Dalton, J. (1808). A New System of Chemical Philosophy (Vol. 1). Manchester: R. Bickerstaff.
- Einstein, A. (1905). Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen. Annalen der Physik, 17, 549–560.
- Thomson, J. J. (1897). Cathode rays. Philosophical Magazine, 44(269), 293–316.
- Perrin, J. (1909). Mouvement brownien et réalité moléculaire. Annales de Chimie et de Physique, 18, 5–114.
- Rutherford, E. (1911). The scattering of α and β particles by matter and the structure of the atom. Philosophical Magazine, 21(125), 669–688.
- Bohr, N. (1913). On the constitution of atoms and molecules. Philosophical Magazine, 26(151), 1–25.
- Schrödinger, E. (1926). Quantisierung als Eigenwertproblem. Annalen der Physik, 79(4), 361–376.
- Chadwick, J. (1932). Possible existence of a neutron. Nature, 129(3252), 312.
- Binnig, G., Rohrer, H., Gerber, C., & Weibel, E. (1982). Surface studies by scanning tunneling microscopy. Physical Review Letters, 49(1), 57–61.