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DictionaryradiumRadioactive element
noun

radium

RAY-dee-um/ˈreɪdiəm/

A radioactive metallic chemical element with atomic number 88.

The element radium has atomic number 88 and is radioactive.

Small sealed radium sample ampoule inside thick transparent shielded container, minimal functional element label Ra on ampoule and radiation trefoil on shield.
Etymology
radLatinray
+iumLatinmetallic element ending

The “ray” root identifies the property that inspired radium’s name: its power to emit energy as rays. The ending places it among names formed for metallic chemical elements.

In context

History That Doesn't Suck15:59
it contains a chemical recently discovered by Marie and Pierre Curie that makes these watch faces glow in the dark called radium
139: From Yeomen (F) to “Hello Girls:” American Women in World War I

More examples

  • Some old watches contain radium in the paint on their dials.
  • Radium occurs naturally in small amounts in uranium ores.
  • Doctors once used radium to treat cancer.