Dmitri Mendeleev organized the chemical elements by recurring chemical properties and used the arrangement to predict missing entries. His account of the periodic law emphasized that gaps and uncertain atomic weights could be tested against similarities within chemical families.[ref]
The Periodic Law of the Chemical Elements by Dmitri Mendeleev. 1889 Faraday Lecture: development of the periodic system and predictions of missing elements
The method was predictive rather than merely a sorted list: a proposed missing element had to fit several properties at once, including the compositions of its compounds. Modern ordering uses atomic number, which distinguishes elemental identity from average isotopic atomic mass. Mendeleev's historical atomic-weight ordering and the modern organizing principle should therefore be kept distinct.
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Dmitri Mendeleev was a Russian chemist best known for developing the Periodic Table of Elements. Born on February 8, 1834, in Siberia, he made significant contributions to the field of chemistry, most notably in 1869 when he published his version of the periodic table. Mendeleev's table organized the known elements by their atomic mass and demonstrated that elements with similar properties appeared at regular intervals, a concept known as periodicity.