I checked & found this in the footnotes:

Although elements 93 and 94, neptunium and plutonium, were created in 1940, their existence was not made public until after the war. They were given provisional names, when they were first made, of “extremium” and “ultimium,” because it was thought impossible that any heavier elements would ever be made.

I try to buy my Mum interesting books for her birthday and Christmas and she always wonders how I find such gems so consistently. My secret - it’s comments like this on Lemmy or the other place, back before the great migration. So thank you - this is going straight on the list! She’ll love it 😊