Periodic law

1869 Fire and matter theory

The periodic law: the properties of the elements turn out to be a periodic function of their atomic weight. Discovered by arranging all elements known in 1869 by increasing weight; the periodicity required gaps in the table for elements not yet found.

The discovery was made by the Russian chemist Dmitri Mendeleev (63 elements, atomic weights in the values settled after Karlsruhe 1860 following Cannizzaro). The first table is dated 17 February 1869; the paper was read to the Russian Chemical Society on 6 March 1869 (by Menshutkin, as the author was ill). Already in his first paper Mendeleev predicted new elements, analogues of aluminium and silicon, and demanded corrections of atomic weights. The predicted eka-aluminium, eka-boron and eka-silicon were found as gallium (1875), scandium (1879) and germanium (1886), and the corrected weights of indium, uranium and beryllium were confirmed.

The German Lothar Meyer arrived independently at a similar system (1864, 1870). The physical explanation of periodicity came with the Englishman Henry Moseley (1913) and quantum mechanics.

Why this date

exact: draft table 17 February (1 March) 1869, paper to the Russian Chemical Society 6 March 1869, printed in Zh. Russ. Khim. Obshch. vol. 1 and Zeitschrift für Chemie 12 (1869)

What it grew from

Chemical atomic theory

influenced · Confidence: medium

The periodic law is the dependence of properties on atomic weight, the quantity introduced by Dalton's atomic theory; but the working values of the weights came from Cannizzaro (1858, Karlsruhe 1860), and Mendeleev explicitly cites them as the basis of the generalisation.

Dispute: Historians tie the periodic law not to Dalton directly but to Cannizzaro's system of atomic weights after Karlsruhe 1860; Dalton's weight values were largely wrong (rule of greatest simplicity), and for the half-century between 1808 and 1860 the table was impossible because of the confusion of atomic and equivalent weights. What comes from Dalton is the concept of atomic weight itself; the missing link is Cannizzaro's work.

Link sources
- Britannica, «Atom», «The beginnings of modern atomic theory» (2024) Verified 20 Sep 2026
  Russian chemist Dmitry Ivanovich Mendeleyev based his system on the atomic weights of the elements as determined by Avogadro's theory of diatomic molecules. In his paper of 1869 introducing the periodic law, he credited Cannizzaro for using 'unshakeable and indubitable' methods to determine atomic weights.
- Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (1971) Verified 20 Sep 2026
  It was the recognition of true atomic weights that permitted Meyer and Mendeleev to formulate the periodic law at the end of the 1860's.
- Mendeleev D., Faraday Lecture 1889 (1889) Verified 20 Sep 2026
  Only such real atomic weights--not conventional ones--could afford a basis for generalisation.
- Kaji M. «D. I. Mendeleev's Concept of Chemical Elements and The Principles of Chemistry», Bull. Hist. Chem. 27 (1), 4-16 (2002), 4
  In pointing out the inconsistency of Gerhardt's atomic weights of metals and arguing that Cannizzaro corrected them with the 'multiatomicity of metals,' Mendeleev clearly recognized Cannizzaro's successful system of atomic weights.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
Spectral analysis

influenced · Confidence: medium

Spectral analysis yielded four new elements (caesium 1860, rubidium 1861, thallium 1861, indium 1863) that entered the 1869 table: the pair Rb, Cs serves Mendeleev as an example of regular weight increments among analogues, while thallium and indium he places wrongly and corrects in 1871; later spectroscopy also yielded gallium (1875), the first confirmation of the predictions. No direct statement by a historian on the influence of spectroscopy on the discovery of the law has been found.

Link sources
- Mendelejeff D., Zeitschrift für Chemie 12, 405-406, 2 (1869) Verified 20 Sep 2026
  Chemically analogous elements have either similar atomic weights (Pt, Ir, Os), or weights which increase by equal increments (K, Rb, Cs).
- Giunta C., 1869 (2024) Verified 20 Sep 2026
  The element which belongs between cadmium (Cd) and tin (Sn) is indium (In), and Mendeleev put indium there in the next version of his table.
- Science History Institute, «Robert Bunsen and Gustav Kirchhoff» (2023) Verified 20 Sep 2026
  These discoveries inaugurated a new era in the means used to find new elements.
- Lente G. «Where Mendeleev was wrong: predicted elements that have never been found», ChemTexts 5 (2019) Verified 20 Sep 2026, quotation assembled from fragments
  the introduction of the spectroscopic method in 1860 ... reduced the incidence of false element identifications
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
Unified system of atomic weights (Cannizzaro, Karlsruhe)

prerequisite for · Confidence: high

Mendeleev arranged the elements by Cannizzaro's atomic weights, which he received at Karlsruhe in 1860, and himself called the establishment of true atomic weights the first condition for the discovery of the periodic law.

Link sources
- Mendeleev D. «The Periodic Law of the Chemical Elements» (Faraday Lecture 1889), J. Chem. Soc. 55, 634-656 (1889) Verified 20 Sep 2026
  In the first place, it was at that time that the numerical value of atomic weights became definitely known. Ten years earlier such knowledge did not exist, as may be gathered from the fact that in 1860 chemists from all parts of the world met at Karlsruhe in order to come to some agreement, if not with respect to views relating to atoms, at any rate as regards their definite representation. Many of those present probably remember how vain were the hopes of coming to an understanding, and how much ground was gained at that Congress by the followers of the unitary theory so brilliantly represented by Cannizzaro. I vividly remember the impression produced by his speeches, which admitted of no compromise, and seemed to advocate truth itself, based on the conceptions of Avogadro, Gerhardt and Regnault, which at that time were far from being generally recognised.
- Kaji M. «D. I. Mendeleev's Concept of Chemical Elements and The Principles of Chemistry», Bull. Hist. Chem. 27 (1), 4-16 (2002), 5
  Along with everyone else in attendance, Mendeleev received a copy of the famous paper on the new atomic-weight system by Stanislao Cannizzaro (1826-1910), who distributed it at the meeting.
- Kaji M. «D. I. Mendeleev's Concept of Chemical Elements and The Principles of Chemistry», Bull. Hist. Chem. (ACS HIST OPA paper) (2005)
  In pointing out the inconsistency of Gerhardt's atomic weights of metals and arguing that Cannizzaro corrected them with the 'multiatomicity of metals,' Mendeleev clearly recognized Cannizzaro's successful system of atomic weights.
- Science History Institute, «Setting the Table» (2019) Verified 20 Sep 2026
  Mendeleev, only 26 years old when he attended the conference in Karlsruhe, made no attempt to hide the fact that his table, based on a common set of atomic masses, was inspired by Cannizzaro's work.
- Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026
  Mendeleev also attended the congress and later wrote of the defense that Cannizzaro had presented for Avogadro's hypothesis.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.
- «Химический конгресс в Карлсруэ», 1860 (1860)

Node sources

«Соотношение свойств с атомным весом элементов», 1, 60-77; Mendelejeff D., Zeitschrift für Chemie 12, 405-406 (1869) Verified 20 Sep 2026
One can predict the discovery of many new elements, for example analogues of Si and Al with atomic weights of 65-75.
Kaji M. «D. I. Mendeleev's Concept of Chemical Elements and The Principles of Chemistry», Bull. Hist. Chem. 27 (1), 4-16 (2002) Verified 20 Sep 2026
February 17, 1869 'An Attempt at a System of the Elements Based on Their Atomic Weight and Chemical Affinity' (the first periodic table)
Mendeleev D. «The Periodic Law of the Chemical Elements» (Faraday Lecture 1889), J. Chem. Soc. 55, 634-656 (1889) Verified 20 Sep 2026
We now know three cases of elements whose existence and properties were foreseen
Stanford Encyclopedia of Philosophy, «Philosophy of Chemistry» (2023) Verified 20 Sep 2026
independently presented periodic tables covering all 63 elements known in 1869.

"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

Periodic law

Why this date

What it grew from

Chemical atomic theory

Spectral analysis

Unified system of atomic weights (Cannizzaro, Karlsruhe)

Other links

Lothar Meyer's system of the elements

Node sources