The breakthrough Dmitri Mendeleev had when constructing the periodic table occurred ________.

Question

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Mendeleev's breakthrough occurred when he arranged the elements in a sequence of increasing atomic mass, which revealed the periodicity of their chemical properties. This laid the foundation for the modern periodic table. The discovery of electrons, protons, and neutrons came later and helped further refine the understanding of the periodic table, but the initial breakthrough was made by Mendeleev's organization of the elements by atomic mass.

The Breakthrough Dmitri Mendeleev Had When Constructing the Periodic Table