Radium

Radium is a very rare metallic chemical element (atomic number 88).  It is very reactive and the pure metal will rapidly oxidize, turning black in color.  All isotopes of radium are highly radioactive.  It was once widely used in industrial applications, but due to the health risks posed to workers, it was completely phased out by the 1960s.  It was also once widely believed to have benefits to human health, but instead the use of radium infused products led to widespread cancer.  Many radium products, such as luminescent watch dials, continue to exist into the 21st century, but as a rule they pose no risk to human health when used as intended.  
Because radium reacts readily with other chemicals, it can easily be absorbed into the human body on exposure, and as a result can effect almost any part of the body.  It is also poorly excreted for the same reason, meaning that a low level exposure could result in ongoing exposure for months or years depending on how it is metabolized.  It often takes the place of calcium in bones.  
Radium at Wikipedia
Category:Chemistry