Inside the atom · models, evidence and changing assumptions

A history of what scientists have assumed, measured and inferred about the atom, from ancient philosophical atomism to electrons, nuclei, quantum states, protons and neutrons, quarks, gluons and precision tests. The focus is how evidence changes the model, rather than a parade of portraits or a claim that one experiment reveals every part at once.

Reading the timeline

Separate by level to compare the chemical atom, electronic structure, quantum states, nuclei, nuclear forces and nucleon structure. Kind tags distinguish a proposal, an observation, an interpretation and a precision test. Each dated milestone includes the assumption challenged, what changed and limits of the inference. Stacked concept cards explain terms and compare historical models with modern understanding. Their modern explanatory language is not attributed to the historical anchor date.

Relationship arrows connect selected evidence, theories and tests. They can mean influence, revision or comparison, as explicitly explained in the source entry; chronological order alone is not a causal claim. Thomson, Nagaoka, Rutherford, Bohr, quantum-mechanical and nuclear models are compared without pretending they were adopted everywhere at once or that every earlier approximation became useless.

What the pictures mean

The cover shows calculated hydrogen probability densities, not photographed electrons. Orbital surfaces and colors are plotting conventions. Nuclear diagrams enlarge the nucleus and simplify its quantum constituents. Three valence-quark circles do not exhaust a proton’s gluon fields or quark–antiquark contributions. Every used illustration has a caption, Commons source and public-domain or Creative Commons credit.

Scope and dating

The selected research extends through the 7 October 2026 thorium nuclear-clock papers. Ancient philosophical dates are approximate. Year-only entries use January 1 only as a coordinate, and experiment, proposal and publication dates are distinguished. This is a substantial survey of atomic structure, not an exhaustive particle-discovery catalogue. Modern theoretical assumptions remain testable; null results are not discoveries of new particles.

All entries are moments or undated concept cards. No continuing model or scientific field is assigned an artificial duration ending today.

Related timeline: Electricity, magnetism and electronics.

Selected primary reading

• Dalton, A New System of Chemical Philosophy, volume 1 (1808)
• J. J. Thomson, Cathode Rays (1897)
• Rutherford, The Scattering of Alpha and Beta Particles and the Structure of the Atom (1911)
• Bohr, On the Constitution of Atoms and Molecules, Part I (1913)
• Schrödinger, Quantisierung als Eigenwertproblem (1926)
• Chadwick, Possible Existence of a Neutron (1932)
• Gell-Mann, A Schematic Model of Baryons and Mesons (1964)92001-3)
• Gross and Wilczek, Ultraviolet Behavior of Non-Abelian Gauge Theories (1973)
• Pohl et al., The Size of the Proton (2010)
• A Thorium-229 Optical Nuclear Clock with Feedback Loop (2026)

Cover image: Calculated hydrogen-state probability-density illustrations, not photographs or electron trajectories.

PoorLeno ( talk ) · Hydrogen Density Plots.png · Public domain. File, author and history No edits beyond thumbnail rendering/scaling.