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Atomic theory

John DaltonAlbert EinsteinJean Perrin

This page delivers no verdict; it presents claims and evidence with their sources. No theory in science is absolute. Method

For everyone

Dalton argued that each element consists of indivisible atoms of characteristic weight and that compounds form when atoms combine in simple ratios. Many scientists regarded atoms only as a useful calculating device until the early 20th century. Einstein's 1905 explanation of Brownian motion and Perrin's measurements of 1908–1909 were decisive in that debate. The idea of atoms also appears in other forms in ancient Greece and in Islamic kalām.

For specialists

The law of multiple proportions (e.g. the 1:2 ratio of oxygen masses combining with carbon in CO and CO₂) was the main support for Dalton's theory. "Energeticists" such as Ostwald and Mach rejected unobservable entities. Einstein linked the mean-square displacement in Brownian motion to Avogadro's number; Perrin measured it consistently by independent methods. Ostwald accepted the existence of atoms in 1909.

Claims

Evidence

Supporting

Measurement

When two elements form more than one compound, the masses of one that combine with a fixed mass of the other are in small whole-number ratios.[1]

Experiment

Perrin measured Avogadro's number with mutually consistent values from Brownian motion, sedimentation equilibrium and diffusion.[3][2]

Challenging

Theoretical argument

In the late 19th century some physicists and chemists found the postulation of directly unobservable atoms unnecessary and unscientific, arguing that thermodynamics could work without them.[4]

Connections

Parent

Chemistry

Builds on

Lavoisier and the conservation of mass

Built upon by

The periodic table

Related

Kalām atomism (al-jawhar al-fard)The Presocratics and ancient atomism

This topic is a stop on these journeys

The story of the atom · stop 4/7

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Sources

  1. BookDalton, J. (1808). A New System of Chemical Philosophy, Part I. R. Bickerstaff, Manchester
  2. ArticleEinstein, A. (1905). Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen. Annalen der Physik 322(8), 549–560DOI 10.1002/andp.19053220806archive
  3. ArticlePerrin, J. (1909). Mouvement brownien et réalité moléculaire. Annales de Chimie et de Physique 18, 5–114
  4. BookNye, M. J. (1972). Molecular Reality: A Perspective on the Scientific Work of Jean Perrin. Macdonald, London

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Cite this page

APA

Athena contributors. (2026). Atomic theory. Athena: The tree of human knowledge. Retrieved November 1, 2026, from https://latifkedi.github.io/athena/en/d/dalton/

BibTeX
@misc{athena_dalton,
  title        = {Atomic theory},
  author       = {{Athena contributors}},
  year         = {2026},
  howpublished = {\url{https://latifkedi.github.io/athena/en/d/dalton/}},
  note         = {Athena: The tree of human knowledge. Accessed: 2026-11-01}
}

Added: September 27, 2026 · Last changed: September 28, 2026 · Page history

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