Which element did Curie find was also radioactive in 1898?

Curie found that thorium was also radioactive in 1898.

Marie Curie was investigating uranium’s mysterious rays when she measured thorium compounds with the sensitive electrometer developed by Pierre Curie and his brother Jacques. Her results showed that thorium emitted radiation spontaneously, like uranium. German physicist Gerhard Carl Schmidt reported the same phenomenon independently earlier in 1898, but Curie’s work helped establish radioactivity as a broader property of matter.

This discovery came during the research that led the Curies to polonium and radium. Curie also recognized that some uranium minerals were more radioactive than uranium itself, suggesting that they contained unknown elements. A common mix-up is to credit Curie with discovering radioactivity itself: Henri Becquerel had discovered uranium’s radiation in 1896. Curie helped expand and name the field, and her work showed that radioactivity came from atoms rather than ordinary chemical reactions.