Question

Why doesn’t helium stay in the atmosphere? Helium gas has a lower density than all other gases in the atmosphere, so naturally, it floats its way into space.

Helium atoms have a half-life that is relatively short. The rate at which helium is leaked into the atmosphere from uranium rich rocks is lower than the decay rate and ergo, helium gas does not stay in the atmosphere.

Due to the much smaller mass of its atoms, helium reaches much higher temperatures than oxygen or nitrogen after absorbing the same amount of solar radiation. With higher temperatures come higher speeds for its atoms, which could easily exceed the escape velocity.

Helium atoms are much lighter than other atmospheric molecules. At a given temperature, a helium atom travels at speeds much closer to the escape velocity than other molecules. At the edge of the atmosphere, a fortuitous collision with other molecules will boost the speed of a helium atom above the escape velocity.

Helium is an expensive non-renewable resource. The high prices made it commercially viable to extract helium from the atmosphere decades ago. By the turn of the 21st century, “cheap” atmospheric helium has been depleted.

Answer 1

Root mean square velocity of any gas is given by

V rms  = (3RT/M)1/2

Since root mean square velocity is inversely proportional to the mass , and as the mass of helium is very light (only 4 amu) , therefore it attains a very large root mean square velocity at room temperature which exceeds the value of escape velocity for palnet earth (11.2km/sec) and hence escapes from out atmosphere.

This is the reason why helium does not stay in the atmosphere - "Helium atoms are very lighter and easily attain a velocity equals to escape velocity and thus escapes from Atmosphere"

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