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It is said that helium is lighter than air. What does that exactly mean ?

If it is lighter than air,then there should be absolutely no trace of helium in earth's atmosphere(it should have flown into the space)

Yet it is one of the most abundant element on earth ?

How is this stored ?. If the balloons filled with helium rise in the air, why don't the helium storage tanks/vehicles rise ?

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    $\begingroup$ the force that makes Helium to rise up is described using this equation: $F=\rho_{air}(h)gV_{helium} - mg$ (its called Archimedes Principle) where $\rho_air$ is air density which depends on height, $g$ is gravitational acceleration $m$ is mass of helium (or helium + container in which it is) $V_{helium}$ is volume of helium (container volume too). notice that Air density gets lower and lower as height increases until it reaches the point where it's zero $\rho gV=mg$ (or even $\rho gV<mg$) and because of that it can't go up. (if it's in container then $mg$ is larger so it wont go up) $\endgroup$ – Gigi Butbaia Apr 26 '14 at 9:20
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    $\begingroup$ Helium is not unique in this. Diatomic hydrogen gas is also "lighter than air" and a candidate for use as a lifting gas. There are, however, certain added risks. $\endgroup$ – dmckee Apr 27 '14 at 0:20
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It is said that helium is lighter than air. What does that exactly mean?

It means that the same volume of helium gas has a lower mass than the same volume of air.

If it is lighter than air,then there should be absolutely no trace of helium in earth's atmosphere(it should have flown into the space)

Well, the gravity of earth affects Helium, too, so there should still be some helium left. More importantly, helium is constantly produced by the alpha decay of radioactive substances.

But it is true that Helium is negligible in the atmosphere, only 5 ppm of the air are Helium.

Yet it is one of the most abundant element on earth?

No, it isn't.

How is this stored? If the balloons filled with helium rise in the air, why don't the helium storage tanks/vehicles rise?

The Helium we exploit is stored under the earth at the same places where also natural gas is stored. It doesn't evade for the same reason why the natural gas doesn't evade: Because it is trapped in the rocks. There's a considerable amount there because it had millions of years to accumulate (recall that it is constantly produced by radioactive decay).

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Yet it is one of the most abundant element on earth ?

It is one of the most abundant elements in the universe, perhaps this is what you are thinking of.

How is this stored ?.

In large quantities it is stored underground in natural wells when the gas or oil has been removed. If the rocks were impervious (airtight) enough to hold natural gas for millions of years they will hold helium. Some will escape, because Helium is so small and so chemically inert it does tend to leak through other materials.

If the balloons filled with helium rise in the air, why don't the helium storage tanks/vehicles rise ?

Helium floats because it is lighter than air. So for every cubic meter of Helium you get a lifting force equal to the difference in weight of air and helium. Since Hair only weighs about 1kg / m3 the most that 1m3 could lift is 1 kg. To get any lift you need a very large volume with a very thin walled tank made of very light material = an airship.

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Yes, Helium is one of the only elements with escape velocity, meaning that once released into the atmosphere, it escapes into space.

In the earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million, it is rare.

Most helium on Earth is a result of radioactive decay. In the heterosphere, a part of the upper atmosphere, helium and other lighter gases are said to be "floating" above the heavy elements.

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    $\begingroup$ "is one of the only elements with escape velocity" Is too strongly stated. All the constituents of the upper atmosphere have some chance of reaching escape speed and are therefore bleeding slowing into space. It is just that the rate is suppressed exponentially in mass so that it is happening enormously more slowly for other gasses than for hydrogen and helium. $\endgroup$ – dmckee Apr 27 '14 at 0:22
  • $\begingroup$ @dmckee: Yeah, you might be right. Once released into the atmosphere, I meant from the surface of earth, not from the upper atmosphere. $\endgroup$ – Immortal Player Apr 27 '14 at 4:49

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