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

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

<p>So how much helium does the decay of these isotopes produce?</p>

<ul>
<li><sup>238</sup>U eventually decays to <sup>206</sup>Pb via the uranium decay series (aka radium series). This decay process ultimately produces eight alpha particles.</li>
<li><sup>235</sup>U eventually decays to <sup>207</sup>Pb via the actinium decay series. This decay process. This decay process ultimately produces seven alpha particles.</li>
<li><sup>232</sup>Th eventually decays to <sup>208</sup>Pb via the thorium decay series. This process ultimately produces six alpha particles.</li>
</ul>

<p>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.</p>

<p>Most of the helium deposits in the Earth are thought to be the result of alpha decay. For example, see <a href="http://www.helium-corp.com/facts/heliumfound.html">Helium Facts: Where Helium is Found</a>.</p>

CC BY-SA 4.0
2018-06-06T11:10:22.983Z
@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.
Luaan
CC BY-SA 3.0
2014-08-23T04:31:01.980Z
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!
f.thorpe
CC BY-SA 3.0
2014-08-23T17:16:15.760Z
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
CC BY-SA 3.0
2014-12-12T07:10:13.090Z
@J.Musser you would need a whole lot of U & Th for that to happen on any reasonable human time scale.
Gimelist
CC BY-SA 3.0
2014-08-23T00:33:21.420Z
2014-12-12T07:15:10.863Z
2014-12-12T07:15:10.863Z
David Hammen
<p>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?</p>

CC BY-SA 3.0
2014-08-22T21:08:39.360Z
Specifically, one wonders about Amarillo, TX gas fields. https://en.wikipedia.org/wiki/National_Helium_Reserve
Deer Hunter
CC BY-SA 3.0
2014-08-22T21:09:10.567Z
@DeerHunter Yeah, that's one.
J. Musser
<earth-history><gas>
How did helium deposits form?
J. Musser
# How did helium deposits form?

(Asked by: username_0 on 2014-08-22)

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?

**Tags:** earth-history, gas

> **Comments:**
> **username_1**: Specifically, one wonders about Amarillo, TX gas fields. https://en.wikipedia.org/wiki/National_Helium_Reserve  
> **username_0**: @username_1 Yeah, that's one.  

---

## 1 answers

### Answer by username_2 on 2014-08-23. Score: 20 | Accepted answer

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](http://www.helium-corp.com/facts/heliumfound.html).

> **Comments:**
> **username_4**: 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!  
> **username_0**: Very good answer. I wonder if this can be replicated in a lab, for commercial use, or if it would cost too much.  
> **username_5**: @username_0 you would need a whole lot of U & Th for that to happen on any reasonable human time scale.  
> **username_3**: @username_0 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.  

---

