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Using his periodic table, Mendeleev predicted the properties of a lot of elements that hadn't been discovered yet. But he missed others - and more than half of his predicted elements were never found!

Two of the most interesting are newtonium and coronium. He predicted both to have atomic masses lighter than hydrogen!

Mysterious green spectal lines had been seen in the Sun's corona in 1869. Mendeleev was not the only one who thought it was a new element: coronium. It later turned out to be a highly ionized form of iron, Fe¹³⁺, meaning that 13 electrons get knocked off by the extremely high temperatures.

Even more interesting is newtonium. Back then, a lot of people thought light consisted of vibrations in a mysterious substance called the aether. In a 50-page paper called “An attempt towards the chemical conception of the aether”, Mendeleev hypothesized that the aether was a noble gas! A bunch of new noble gases had just been discovered, forcing a new column in his table. He dubbed this one newtonium, and he estimated its atomic mass as 0.17.

Details here:

• Gábor Lente, Where Mendeleev was wrong: predicted elements that have never been found, https://link.springer.com/article/10.1007/s40828-019-0092-5

Moral: you don't have to be right all the time.