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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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Helium (He) is formed by radioactive decay of uranium (U) and thorium (Th) to lead (Pb).

There's another name for the nucleus of a 4He 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, 238U decays to 234Th via alpha decay. There are three key isotopes that lead to the generation of helium: 238U, 235U, and 232Th. 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?

  • 238U eventually decays to 206Pb via the uranium decay series (aka radium series). This decay process ultimately produces eight alpha particles.
  • 235U eventually decays to 207Pb via the actinium decay series. This decay process. This decay process ultimately produces seven alpha particles.
  • 232Th eventually decays to 208Pb 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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  • $\begingroup$ 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! $\endgroup$ – farrenthorpe Aug 23 '14 at 4:31
  • $\begingroup$ Very good answer. I wonder if this can be replicated in a lab, for commercial use, or if it would cost too much. $\endgroup$ – J. Musser Aug 23 '14 at 17:16
  • $\begingroup$ @J.Musser you would need a whole lot of U & Th for that to happen on any reasonable human time scale. $\endgroup$ – Gimelist Dec 12 '14 at 7:10
  • $\begingroup$ @J.Musser We have better ideas - though even those aren't commercially viable right now. One is of course nuclear fusion, which could produce helium as a byproduct - though given how efficient fusion can be, I'm not sure if it could compare to our current use even if we produced all energy from fusion. Second is the alpha decay of other radioactive materials, with a shorter half-life (e.g. Polonium-212) - the tricky bit is finding a source material that's easy to produce. We will probably find some reasonable solutions eventually - nuclear research is rather restricted. $\endgroup$ – Luaan Jun 6 '18 at 11:10

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