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How did the discoveries of gallium, scandium, and germanium support Mendeleev’s periodic law?

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Periodic table of the elements with color-coded categories and labeled element groups, showing atomic numbers, symbols, and basic information for each chemical element. — periodic table Modern version of the periodic table of the elements. To learn an element's name, atomic number, electron configuration, atomic weight, and more, select one from the table.

Dmitri Mendeleev’s periodic law proposed that elements arranged by increasing atomic weight exhibit a clear periodicity of properties. While initially met with limited interest, the law gained widespread scientific acceptance following the discovery of gallium (1875), scandium (1879), and germanium (1886).12

These discoveries served as powerful evidence for the validity of Mendeleev’s system because he had used his periodic table to predict both the existence and the specific properties of these then-unknown elements. By leaving vacant spaces in his table for elements that had not yet been identified, Mendeleev demonstrated that his arrangement was not merely coincidental but reflected a fundamental order in nature. The subsequent identification of these elements confirmed his predictions, transforming the periodic table into the essential framework for modern chemical theory.12