How many elements were known when Mendeleev published the periodic table?

When Dmitri Mendeleev presented his periodic table in 1869, only 63 elements had been discovered. His key insight was to arrange elements so that those with similar chemical behavior lined up, even when that meant leaving gaps for elements not yet found. He predicted the existence and properties of several unknown elements, including gallium, scandium, and germanium, which were later discovered and matched his predictions closely. The table became a powerful tool because it showed that elemental properties repeat in a regular pattern. Modern chemistry still builds on Mendeleev’s organizing principle, though atomic number now defines the order.