Dmitri Ivanovich Mendeleev (8 February 1834–2 February 1907, Gregorian calendar) was a Russian scientist best known for formulating the periodic law and publishing an influential periodic table in 1869. He organized chemical elements by their atomic weights and recurring properties, leaving gaps for elements not yet discovered. His predictions helped establish periodic classification as a central framework of chemistry. Alongside this work, he contributed to scientific teaching, industrial research, and the standardization of weights and measures. (vniim.ru)
Early life and academic career
Mendeleev was born in the Tobolsk region of Siberia, into a large family. His early life was disrupted by his father’s death and the loss of the family’s livelihood. His mother subsequently took him to Saint Petersburg to pursue his education. He entered the Main Pedagogical Institute in 1850 and graduated in 1855, initially qualifying for work as a teacher. He later taught and conducted research at Saint Petersburg University, becoming professor of general chemistry in 1867. (periodic-table.rsc.org)
Between 1859 and 1861, Mendeleev worked abroad, particularly in Heidelberg. He attended the Karlsruhe chemical congress in 1860, an important meeting in efforts to clarify atomic and molecular weights. After returning to Russia, he published a Russian-language textbook on organic chemistry in 1861. His later textbook, The Principles of Chemistry, became closely associated with the development and presentation of his periodic system. Teaching required him to find a coherent way to explain relationships among elements, rather than treating their properties as disconnected facts. (chem.msu.ru)
Development of the periodic system
Mendeleev’s work emerged from a wider nineteenth-century search for order among the elements. Earlier investigators had identified small groups of chemically similar substances or proposed arrangements based on atomic weight. John Newlands described a “law of octaves,” while Julius Lothar Meyer independently developed classifications resembling Mendeleev’s. More consistent atomic-weight values, discussed at Karlsruhe, made these comparisons increasingly productive. Mendeleev was therefore not the sole originator of periodic classification, but his systematic development and defense of it were especially influential. (rsc.org)
In 1869, while preparing his textbook, Mendeleev compared atomic weights, formulas, and chemical properties. His first periodic scheme was dated 17 February in the Julian calendar then used in Russia, equivalent to 1 March in the Gregorian calendar. Its organizing principle was that elemental properties recur as atomic weight increases. An improved arrangement appeared in 1871, giving a clearer expression to the periodic law and to relationships among chemical families. (edu.rsc.org)
Atomic weight—closely corresponding to what is now called relative atomic mass—was not an inflexible sorting rule. Where numerical order conflicted with chemical resemblance, Mendeleev prioritized the latter. He placed tellurium before iodine despite their atomic weights suggesting the reverse order. He also proposed revisions to some accepted atomic weights. These decisions distinguished his system from a simple numerical list: the arrangement could identify inconsistencies in existing measurements as well as organize accepted information. (edu.rsc.org)
Predictions and experimental confirmation
Mendeleev’s most consequential step was leaving spaces for missing elements and predicting their properties from neighboring entries. In 1871 he described hypothetical substances called eka-boron, eka-aluminium, and eka-silicon. These were subsequently associated with scandium, gallium, and germanium. The discoveries of these elements by 1886 provided substantial support for his classification. His predictions concerned not only atomic weights but also physical properties and the composition and behavior of compounds. (rsc.org)
For eka-aluminium, for example, he predicted an atomic weight of about 68, a solid density near 6 grams per cubic centimetre, and a low melting point. Gallium, discovered in 1875, closely matched these expectations. He also anticipated the formula of its oxide and aspects of its chemical behavior. Such comparisons gave the periodic system an experimentally testable character: it could guide investigation of unknown substances rather than merely summarize known ones. (periodic-table.rsc.org)
Other research and public service
Mendeleev’s research extended beyond classification. His studies of liquids included investigation of what he called an “absolute boiling point,” related to the modern concept of the critical temperature. He also examined mixtures of alcohol and water, and worked on problems involving gases and liquid properties. These interests connected his experimental work with questions subsequently developed within physical chemistry. (chem.msu.ru)
He investigated petroleum processing, conducted agricultural experiments involving fertilizers, and participated in projects concerning shipbuilding and smokeless gunpowder. In 1892 he became scientific custodian of Russia’s standards depot, and from 1893 until his death he directed the reorganized Main Chamber of Weights and Measures. His work in metrology included improving measurement standards and comparing Russian units with foreign and metric measures. (rsc.org)
Later interpretation and recognition
Mendeleev’s original system preceded a detailed understanding of atomic structure. In 1913, after his death, Henry Moseley’s research established atomic number as the appropriate basis for ordering elements. Modern classification consequently uses the number of protons in the atomic nucleus, rather than atomic weight. This explains apparent exceptions such as tellurium and iodine. Later work on electrons, electron configurations, and quantum mechanics supplied physical explanations for recurring chemical properties. (periodic-table.rsc.org)
The discovery of the noble gases also required extending the original classification, while preserving its underlying periodic relationships. Mendeleev never received a Nobel Prize in Chemistry. His name was instead commemorated in mendelevium, the chemical element with atomic number 101. (periodic-table.rsc.org)