How Mendeleev Predicted Missing Elements

⚙️ How Mendeleev Predicted Missing Elements

Mendeleev's genius lay in treating gaps not as failures but as predictions. When he noticed that placing elements in order of atomic weight caused some elements to fall in groups where their properties did not fit, he made the bold decision to trust the pattern of properties over the order of mass. He swapped tellurium and iodine because iodine behaved chemically like fluorine, chlorine and bromine — and that mattered more to him than the strict mass order. For the gaps, he used the surrounding known elements to calculate what the missing element should look like. If an element was between silicon (mass 28) and tin (mass 118) in Group 4, the unknown element should have intermediate properties. His "eka-silicon" prediction included the atomic weight (72), density (5.5 g/cm³), and that it would form a white oxide with formula XO₂. When germanium was isolated in 1886, it matched every prediction almost exactly. This is the hallmark of a good scientific theory: it makes testable predictions that are then confirmed by experiment.