Which is lightest metal in the world?
It is lithium (Li). Its density at a room temperature is `0.533 g/(cm^3)` (almost twice a light as water). Actually, it is less dense than kerosene which is used to store sodium and potassium.
Lithium is an alkali metal, although the least active. It has many interesting properties. There is relatively large amount of lithium on Earth and it is widely used in technology.