The periodic table
Mendeleev arranges the elements by weight and leaves gaps for ones not yet found.
- When
- Known to
- ± 1 year
- Era
- Industrial, Holocene
Mendeleev publishes his table in 1869 and does something bolder than classification: where the pattern requires an element that nobody has seen, he leaves a space and predicts its properties. Gallium, scandium and germanium are found within fifteen years, matching closely. The underlying reason for the periodicity — electron shells — is not understood for another sixty years.