The chemical symbol for radium is "Ra." It is a radioactive alkaline earth metal that was discovered by Marie Curie and her husband Pierre Curie in 1898. Radium is well-known for its luminescent properties and has historically been used in certain medical applications, notably in cancer treatment. Its radioactivity can produce radon gas as a decay product, which poses health risks. Due to its hazardous nature, the use of radium has declined significantly in recent times, replaced by safer, non-radioactive alternatives in various applications.