Radium (Ra, 88, December 1989)

Discovered by Marie Curie when she noticed that there was some yet unknown more radioactive element in their raw samples, after uranium and polonium, which she published 6 months prior, had already been separated. Published on December 1989 as: Section "Sur une nouvelle substance fortement radio-active, contenue dans la pechblende".

The uranium 238 decay chain is the main source of naturally occurring radium.