Another four elements

Helium

Helium is twice as heavy as hydrogen, but it is still only one seventh the weight of air. Unlike hydrogen does not burn, so it is used in modern airships.

Lithium
Lithium was discovered in 1817 by Swedish scientists Johan August Arfvedson. It takes its name from the Latin word for rock, although it is actually a metal. Lithium is so light (42 times lighter than the heaviest element, osmium) and so soft that it can be easily cut with a knife. It floats because it is half as heavy as water, and lighter than some types of wood. It is used to make lithium batteries.

Potassium and sodium
Both are discovered in 1807 by Sir Humphry Davy. Both are metals that are lighter than water. In a laboratory, potassium is usually kept in paraffin because if it comes into contact with water it releases hydrogen and generate so much heat that it catches fire.