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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 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.

The periodic law states that properties of chemical elements recur systematically when the Elements are ordered by atomic number. Similar outer electron configurations help explain recurring chemical behavior. For example, lithium, sodium and potassium belong to the same periodic group and commonly form singly charged positive ions.^[ref]

Dmitri Mendeleev originally organized the pattern largely using relative atomic mass and chemical similarities. His 1889 lecture recounts predictions of undiscovered elements from gaps in the arrangement. The modern atomic number formulation supplies a more fundamental ordering than mass alone; the historical predictions show that the scheme could constrain unknown chemistry, rather than merely list known substances.^[ref]