History of the periodic table

The periodic table grew from the work of many scientists: first by finding patterns in chemical properties, then by explaining those patterns through atomic structure.

This timeline follows the development from early element discovery to the organization and naming of elements 1–118.

1. From element discovery to periodic patterns

Date	Milestone
1669	Hennig Brand isolates phosphorus from heated urine residues. Element discovery grew from practical experiments and alchemy; Robert Boyle later investigated phosphorus too.
1789	Antoine Lavoisier groups simple substances by their properties, an early step towards systematic classification.
1829	Johann Döbereiner identifies chemically similar groups of three elements, known as triads.
1862	Alexandre-Émile Béguyer de Chancourtois arranges elements by atomic weight around a cylinder, revealing recurring similarities.
1864–1865	John Newlands develops his law of octaves: chemical similarities recur in an ordering by atomic weight.
1869–1870	Dmitri Mendeleev publishes his periodic system in 1869. Julius Lothar Meyer, working independently, also develops periodic classifications and publishes his mature arrangement in 1870.
1875–1886	Discoveries of gallium, scandium and germanium support Mendeleev’s predictions for gaps in the table.

Why Mendeleev’s table mattered

Mendeleev used chemical similarities as well as atomic weights. He left gaps for undiscovered elements and predicted their properties, allowing later discoveries to test the arrangement. His achievement built on earlier work and developed alongside Meyer’s independent research.

A successful table did more than catalogue known substances: it helped chemists identify what was still missing.

2. Discovering the structure behind the table

Date	Milestone
1894	Lord Rayleigh and William Ramsay identify argon in air. Further noble-gas discoveries follow; these elements are now in group 18.
1896	Henri Becquerel discovers spontaneous radiation from uranium compounds, opening the study of radioactivity.
1897	J. J. Thomson identifies the electron through cathode-ray experiments, showing that atoms contain smaller particles.
1898	Marie and Pierre Curie announce polonium and radium. Separately, Ramsay and Morris Travers discover neon, krypton and xenon.
1902–1903	Ernest Rutherford and Frederick Soddy develop the radioactive-transformation theory: radioactive atoms can change into atoms of other elements.
1911	Rutherford proposes the nuclear model, interpreting scattering experiments by Hans Geiger and Ernest Marsden. Most of an atom’s mass and positive charge are concentrated in a small nucleus.
1913	Niels Bohr introduces quantized energy levels in a model of the hydrogen atom. This was an important step, rather than the final quantum-mechanical description.
1913	Henry Moseley’s X-ray measurements establish atomic number as the basis for ordering elements, resolving exceptions in a simple atomic-weight ordering.
1913	Soddy introduces the term isotope for atoms occupying the same place in the periodic table despite differing in mass.
1919	Rutherford’s nuclear experiments identify hydrogen nuclei as constituents of heavier nuclei, contributing to the identification of the proton.

3. Explaining and extending the modern table

Date	Milestone
1920s	Quantum mechanics provides a deeper explanation of electron states and periodic patterns. Electron orbitals replace the idea that electrons follow fixed planetary paths.
1932	James Chadwick discovers the neutron, explaining how atoms of one element can differ in mass. In separate accelerator work, John Cockcroft and Ernest Walton split lithium nuclei into helium nuclei.
1944–1945	Glenn T. Seaborg develops the actinide concept in 1944; a table incorporating it is published in 1945. It places the actinides in a series analogous to the lanthanides and helps organize the transuranium elements.
2016	IUPAC approves nihonium (113), moscovium (115), tennessine (117) and oganesson (118), completing the permanent naming of the seventh period.

From atomic weight to atomic number

Early tables mainly used relative atomic weights and chemical similarities. The modern table follows atomic number: the number of protons in the nucleus. Isotopes of one element have different neutron counts but share the same atomic number and place in the table.

The lanthanides and actinides are often drawn beneath the main table to keep it compact; they remain part of the periodic system.