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How did Dmitry Mendeleyev predict the properties of gallium?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — boron group element Modern version of the periodic table of the elements (printable).

Dmitry Mendeleyev predicted the existence and properties of gallium by identifying gaps in his periodic table. While organizing the elements based on their atomic weight and chemical properties—specifically paying close attention to valence—Mendeleyev noticed that certain elements were missing from the established sequence.12

He referred to the undiscovered element located between aluminum and indium as "eka-aluminum". By analyzing the properties of the elements surrounding this gap, he was able to accurately predict the characteristics of the unknown metal. When Paul-Émile Lecoq de Boisbaudran discovered gallium in 1875, its observed properties matched those Mendeleyev had previously described. This successful prediction provided critical evidence for the validity of the periodic law and led to its widespread acceptance in the scientific community.3412