Knowra History of the periodic table History of the periodic table The development of ways to classify chemical elements, culminating in the modern periodic table ordered by atomic number and recurring chemical properties.
Antoine Lavoisier : An eighteenth-century French chemist whose work helped establish modern chemistry and systematic element classification. His 1789 list grouped known elements into gases, metals, nonmetals, and earths.
Atomic number : The number of protons in an atom's nucleus, which uniquely identifies a chemical element. Ordering elements by proton count resolved conflicts in weight-based arrangements.
Gallium : A metallic element with atomic number 31, discovered in 1875 and predicted by Mendeleev as eka-aluminium. Its properties closely matched Mendeleev's published prediction, validating his approach.
Atomic weight : The weighted average mass of an element's atoms, accounting for the natural abundances of its isotopes. Early tables used atomic weight, but it could not consistently determine element order.
Johann Wolfgang Döbereiner : A German chemist who identified triads of elements with related properties and approximately related atomic weights. His triads were an early clue that element properties recur in ordered sequences.
Periodic law : The principle that elemental properties recur periodically when elements are arranged by increasing atomic number. It states the regularity that the table's rows and columns encode.
Germanium : A metalloid element with atomic number 32, discovered in 1886 and predicted by Mendeleev as eka-silicon. Its measured properties closely confirmed a second prediction from the table.
Mendeleev's periodic table : Dmitri Mendeleev's 1869 arrangement of elements by atomic weight, chemical properties, and gaps for predicted elements. It exemplifies the influential historical table before atomic number became the ordering principle.
Alexandre-Émile Béguyer de Chancourtois : A French geologist who arranged elements by atomic weight on a cylindrical chart called the telluric screw. His 1862 chart placed similar elements in recurring vertical alignments.
Henry Moseley : An English physicist whose X-ray measurements established atomic number as the basis for ordering elements. His 1913 results supplied experimental evidence for the modern ordering rule.
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