Beryllium

Beryllium is a metallic element (atomic number 4).  Like lithium, it is very light, but because of it's different atomic structure, it is far less reactive in water and oxygen.  As such, it is used in applications where stability and light weight is important.  However, because it is rare and expensive, it has limited practical uses, but is particularly important in aerospace uses.  It is otherwise not used widely in commercial uses because it is both toxic and many people are allergic to it.
Beryllium is often alloyed to other metals to make them lighter and more corrosion resistant.  
In medicine, beryllium is used in X-ray machines as it does not absorb x-rays as much as other metals and therefore stays cool when it is used to direct the radiation.  
Beryllium at Wikipedia
Category:Chemistry