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Why do radium compounds glow faintly in the dark?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkaline-earth metal Modern version of the periodic table of the elements (printable).

Radium compounds exhibit a faint bluish glow in the dark due to their intense radioactivity. This phenomenon occurs because the alpha particles emitted by the radium excite electrons within the other elements present in the compound. When these excited electrons subsequently return to their lower energy states, they release that energy in the form of visible light.1

This process is a form of radioluminescence. Historically, this property was utilized in self-luminous paints by mixing radium with a phosphor, such as zinc sulfide, which would glow when bombarded by the radiation. However, the use of radium in such applications was discontinued after the severe health risks associated with radium exposure—such as anemia and bone cancer—were recognized.12