Answer the following question given this paragraph: 

Most of the elements we know about do not exist freely in nature. Sodium cannot be found by itself (unless we prepare it in the laboratory) because it interacts easily with other materials. On the other hand, the element helium does not interact with other elements to any extent. We can isolate helium from natural gas during the process of drilling for oil.


Q: Sodium is an example of what sort of thing that cannot exist by itself in nature?


A:
element