ionium

The claimed element ionium (symbol Io), before it was realised this was actually thorium-230.

A related method is ionium-thorium dating, which measures the ratio of ionium (thorium-230) to thorium-232 in ocean sediment.

Likewise, both thorium and ionium are assumed to precipitate out in a constant ratio; no chemical process favors one form over the other.

(Th was once known as ionium, before it was realised it was the same element.)

There, in the space of a few months, using extremely primitive apparatus, Hahn discovered mesothorium I, mesothorium II and - independently from Bertram Boltwood - the mother substance of radium, ionium.

Ionium was a name given early in the study of radioactive elements to the Th isotope produced in the decay chain of U before it was realized that ionium and thorium are chemically identical.