Davyum

Davyum was the name given to a purported chemical element by Russian chemist Dimitri Mendeleev in 1869. Mendeleev, the creator of the periodic table, predicted the existence of several undiscovered elements based on gaps in his table. He named these elements using the Sanskrit prefixes eka- (one) and dvi- (two) attached to the name of the element above them in the same group. Thus, Davyum, derived from the Russian name for sodium (natrium, Na), was assigned the provisional name eka-caesium (or eka-cesium), meaning it was predicted to lie directly below caesium (Cs) in the alkali metal group of the periodic table.

Mendeleev estimated Davyum's atomic weight and other properties based on periodic trends. His predictions spurred numerous attempts to isolate and identify the element. However, no element with the predicted properties was ever discovered.

The element that would eventually occupy the space below caesium in the alkali metal group is francium (Fr), discovered by Marguerite Perey in 1939. Francium is a highly radioactive element, and its properties differ significantly from Mendeleev's predictions for Davyum. It's now understood that Mendeleev's predictions, while insightful, were not completely accurate due to the complexity of the electronic structure of heavy elements and relativistic effects. The name Davyum is therefore considered obsolete and has no place in modern chemistry. It remains a historical footnote in the development of the periodic table.