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I always wondered, because helium gas is so lightweight, how did it get into the deposits like the ones that people tap? Is it formed underground, or was it trapped?

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Specifically, one wonders about Amarillo, TX gas fields. en.wikipedia.org/wiki/National_Helium_Reserve –  Deer Hunter Aug 22 at 21:08
@DeerHunter Yeah, that's one. –  J. Musser Aug 22 at 21:09

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There's another name for the nucleus of a helium-4 atom: It's an alpha particle. Alpha decay is one of the pathways by which radioactive elements eventually decay into non-radioactive elements. For example, uranium-238 decays to thorium-234 via alpha decay. There are three key isotopes that lead to the generation of helium: Uranium-238, uranium-235, and thorium-232. Each of these is a "primordial" radioactive isotope. they were present when the Earth formed, and their half-lives are sufficiently long so that a good fraction of that original content has not yet decayed.

So how much helium does the decay of these isotopes produce?

  • Uranium-238 eventually decays to lead-206 via the uranium decay series (aka radium series). This decay process ultimately produces eight alpha particles.
  • Uranium-235 eventually decays to lead-207 via the actinium decay series. This decay process. This decay process ultimately produces seven alpha particles.
  • Thorium-232 eventually decays to lead-208 via the thorium decay series. This process ultimately produces six alpha particles.

After the alpha decay, the parent nuclide suddenly has an excess of two electrons orbiting it. The alpha particle has a deficit of two electrons. The alpha particle quickly grabs those two excess electrons and becomes a helium atom.

Most of the helium deposits in the Earth are thought to be the result of alpha decay. For example, see Helium Facts: Where Helium is Found.

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Yes because any helium that would've existed during the formation of the planet would have escaped the atmosphere by now. Argon is heavy enough though to have stuck around! –  farrenthorpe Aug 23 at 4:31
Very good answer. I wonder if this can be replicated in a lab, for commercial use, or if it would cost too much. –  J. Musser Aug 23 at 17:16

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