Ionisation energy of promethium fills one of the last holes in periodic table

The ionisation energy of promethium has been determined experimentally for the first time, solving an almost 75-year-old mystery and filling in one of the last gaps in the periodic table. Promethium, element 61, only exists in trace quantities on Earth and has to be synthesised through nuclear reactions. However, since its discovery at Oak Ridge, US, in 1945, no team has managed to measure its ionisation energy – the minimum amount of energy required to remove its most loosely bound electron – through experiments.