The lightest gas is hydrogen. This element is lighter than air and was used in dirigibles, or blimps, until deemed unsafe. Hydrogen is in the topmost layers of Earth's atmosphere because it rises above other gases.
Hydrogen is light because it is the least-massive element on the periodic table. Hydrogen has a single proton and a single electron, but only the proton gives it any significant mass. Hydrogen weighs only 1 atomic mass unit.
Only objects such as stars and giant planets such as Jupiter with massive gravitational pulls can hold onto this lightest of elements. In Earth's atmosphere, hydrogen atoms collide with atoms of other gases. These collisions provide enough force to hurl light hydrogen atoms into space; therefore, hydrogen is found only in trace amounts in Earth's atmosphere.