<p>The latter.</p>
<p>To astronomers, a metal is any element that is <em>not</em> hydrogen or helium, because these elements together constitute most of the elements in the Universe, by far.</p>
<p>This means that, in many circumstances all other elements can be neglected, at least to first order.</p>
<p>By mass, H and He account for some 74% and 24% in the present-day Universe, respectively, while the next-most abundant elements are on the &lt;1% scale. And because these elements are also heavier, if you consider the abundance <em>by number</em>, H and He account for roughly 99.9% of all atoms.</p>
<p>Lithium, on the other hand, only account for a few billionths of all atoms. Here's an overview of the abundances in the Solar System, in terms of <em>number</em> densities, i.e. not mass densities:</p>
<p><a href="https://i.sstatic.net/2MDt6.png" rel="noreferrer"><img src="https://i.sstatic.net/2MDt6.png" alt="A" /></a>
<sup>Credit: <a href="https://commons.wikimedia.org/wiki/File:SolarSystemAbundances.png" rel="noreferrer">Wikimedia Commons user 28bytes, under C.C.-by-S.A.-3.0</a>.</sup></p>
<p>The reason for the zig-zag pattern is that, in general, elements with an even number of protons are more stable than those with an odd number, as discussed in <a href="https://astronomy.stackexchange.com/q/39561/5264">this post</a>.</p>
<p>Note that the <span class="math-container">$y$</span> axis is logarithmic, i.e. each tick mark denotes a ten-fold increase in abundance. The axis is arbitrarily scaled to <span class="math-container">$10^6$</span> silicon (Si) atoms, so the Si dot is at <span class="math-container">$\log A \equiv 6$</span>. Oxygen, for instance, is at <span class="math-container">$y \simeq 7.15$</span>, so it is <span class="math-container">$10^{7.15-6}\simeq14$</span> times more abundant than Si.</p>
<h3>Are we wrong?</h3>
<p>The terminology used by astronomers may be considered wrong by other physicists and chemists. But why do we use the term at all? To distinguish some of the elements from other elements, based on their properties, I'd say. In astronomy, it very often makes sense to consider the elements of the Universe as divided in three:</p>
<ul>
<li>hydrogen, which is most of the mass and most of the atoms, and which provides most of the energy in stars,</li>
<li>helium, which adds significantly to the mass, lowering the mass fraction of hydrogen, and</li>
<li>all other elements, which together help cooling the gas (so it can condense), form dust grains (altering observed spectra), have a very roughly equal amount of protons and neutron (so <span class="math-container">$A\simeq2Z$</span>), are formed in other processes than the Big Bang (except for a small part of the lithium), etc.</li>
</ul>
<p>In physics, you may be more concerned with the material's ability to conduct electricity and hence consider sodium a metal until the pressure becomes too high, after which you call it a non-metal.</p>
<p>In chemistry, you may be more interested in the materials ability to form bonds, and hence do not consider arsenic and antimony metals.</p>
<p>So, I think you can say that there isn't a formal definition of the term &quot;metal&quot; — it depends on the context. From <a href="https://en.wikipedia.org/wiki/Metal" rel="noreferrer">Wikipedia</a>:</p>
<blockquote>
<p>Around 95 of the 118 elements in the periodic table are metals (or are
likely to be such). The number is inexact as the boundaries between
metals, nonmetals, and metalloids fluctuate slightly due to a lack of
universally accepted definitions of the categories involved</p>
</blockquote>
<p>Apart from these practical reasons, there are also historical reasons, as discussed in <a href="https://astronomy.stackexchange.com/a/12051/5264">this answer</a> by Rob Jeffries.</p>

CC BY-SA 4.0
2020-10-27T12:03:00.763Z
2020-10-29T10:36:08.623Z
2020-10-29T10:36:08.623Z
pela
<p>My lecture notes are conflicting on this issue so I seek definitive clarification. In Astronomy, are metals either</p>
<p><strong>A:</strong> the <em>elements which form within stars</em> (i.e., not Big Bang <em>nucleosynthesis</em>, where most of the lithium (Li) was formed as far as I know)</p>
<p>or</p>
<p><strong>B:</strong>  <em>everything heavier than helium</em> (He)?</p>

CC BY-SA 4.0
2020-10-27T21:11:35.660Z
Obligatory https://xkcd.com/2340/ and https://explainxkcd.com/2340/
coblr
CC BY-SA 4.0
2020-10-28T02:36:25.557Z
Also related: https://i.sstatic.net/aLu3K.png
whatsisname
CC BY-SA 4.0
2020-10-28T08:05:53.307Z
@whatsisname I thought it was just a meme, never knew it was considered so in reality! https://astronomy.swin.edu.au/cms/cpg15x/albums/userpics/PeriodicTable.gif
varun
CC BY-SA 4.0
2020-10-28T20:39:41.170Z
Of course, lithium is considered a metal in pretty much every field of science.  A better question would be to ask about *carbon*, the first element where astronomers would disagree with other scientists about being it a metal...
Darrel Hoffman
CC BY-SA 4.0
2020-10-29T00:37:06.760Z
@Darrel Hoffman: *"[Graphene](https://en.wikipedia.org/wiki/Graphene) conducts heat and electricity very efficiently along its plane."*. And *"[...the highest thermal conductivities of all known materials](https://en.wikipedia.org/wiki/Carbon)"*. The same goes for [carbon nanotubes](https://en.wikipedia.org/wiki/Carbon_nanotube) - *"thermal conductivity along its axis of about 3500 W·m−1·K−1; compare this to copper, a metal well known for its good thermal conductivity, which transmits 385 W·m−1·K−1."*
Peter Mortensen
CC BY-SA 4.0
2020-10-29T00:44:20.067Z
@darrel-hoffman that wouldn't really be a "better question", because this question is specifically about Lithium, as the third lightest element, and which according to the OP was produced in quantity during the big bang. Why would helium be considered a nonmetal (astronomically) but not lithium?
Stephen Holt
CC BY-SA 4.0
2020-10-29T13:27:04.567Z
@StephenHolt Because there's no disagreement between scientific fields that lithium is a metal, and that helium and hydrogen are not.  Carbon is the first element whose metal-ness is defined differently by astronomers and other scientists.
Darrel Hoffman
CC BY-SA 4.0
2020-10-29T16:39:57.100Z
@DarrelHoffman Hydrogen can be a metal. https://en.wikipedia.org/wiki/Metallic_hydrogen
DavePhD
CC BY-SA 4.0
2023-03-04T16:52:59.583Z
@DarrelHoffman - right, so everyone agrees that lithium is a metal. But that in itself provides the answer to the OP's question. For astronomers, everything heavier than helium is a metal, including lithium. Case closed. You're quite right that carbon is the first element where there's some disagreement, but I assume the OP already knew that. Cheers.
Stephen Holt
<terminology><nucleosynthesis><metal>
Is lithium considered a metal in astronomy?
zabop
# Is lithium considered a metal in astronomy?

(Asked by: username_0 on 2020-10-27)

My lecture notes are conflicting on this issue so I seek definitive clarification. In Astronomy, are metals either

**A:** the *elements which form within stars* (i.e., not Big Bang *nucleosynthesis*, where most of the lithium (Li) was formed as far as I know)

or

**B:** *everything heavier than helium* (He)?

**Tags:** terminology, nucleosynthesis, metal
> **Comments:**
> **username_1**: Obligatory https://xkcd.com/2340/ and https://explainxkcd.com/2340/  
> **username_2**: Also related: https://i.sstatic.net/aLu3K.png  
> **username_3**: @whatsisname I thought it was just a meme, never knew it was considered so in reality! https://astronomy.swin.edu.au/cms/cpg15x/albums/userpics/PeriodicTable.gif  
> **username_4**: Of course, lithium is considered a metal in pretty much every field of science.  A better question would be to ask about *carbon*, the first element where astronomers would disagree with other scientists about being it a metal...  
> **username_5**: @Darrel Hoffman: *"[Graphene](https://en.wikipedia.org/wiki/Graphene) conducts heat and electricity very efficiently along its plane."*. And *"[...the highest thermal conductivities of all known materials](https://en.wikipedia.org/wiki/Carbon)"*. The same goes for [carbon nanotubes](https://en.wikipedia.org/wiki/Carbon_nanotube) - *"thermal conductivity along its axis of about 3500 W·m−1·K−1; compare this to copper, a metal well known for its good thermal conductivity, which transmits 385 W·m−1·K−1."*  
> **username_6**: @darrel-hoffman that wouldn't really be a "better question", because this question is specifically about Lithium, as the third lightest element, and which according to the OP was produced in quantity during the big bang. Why would helium be considered a nonmetal (astronomically) but not lithium?  
> **username_4**: @StephenHolt Because there's no disagreement between scientific fields that lithium is a metal, and that helium and hydrogen are not.  Carbon is the first element whose metal-ness is defined differently by astronomers and other scientists.  
> **username_7**: @DarrelHoffman Hydrogen can be a metal. https://en.wikipedia.org/wiki/Metallic_hydrogen  
> **username_6**: @DarrelHoffman - right, so everyone agrees that lithium is a metal. But that in itself provides the answer to the OP's question. For astronomers, everything heavier than helium is a metal, including lithium. Case closed. You're quite right that carbon is the first element where there's some disagreement, but I assume the OP already knew that. Cheers.  

---

## 1 answers

### Answer by username_8 on 2020-10-27. Score: 44 | Accepted answer

The latter.

To astronomers, a metal is any element that is *not* hydrogen or helium, because these elements together constitute most of the elements in the Universe, by far.

This means that, in many circumstances all other elements can be neglected, at least to first order.

By mass, H and He account for some 74% and 24% in the present-day Universe, respectively, while the next-most abundant elements are on the <1% scale. And because these elements are also heavier, if you consider the abundance *by number*, H and He account for roughly 99.9% of all atoms.

Lithium, on the other hand, only account for a few billionths of all atoms. Here's an overview of the abundances in the Solar System, in terms of *number* densities, i.e. not mass densities:

[![A](https://i.sstatic.net/2MDt6.png)](https://i.sstatic.net/2MDt6.png)
Credit: [Wikimedia Commons user 28bytes, under C.C.-by-S.A.-3.0](https://commons.wikimedia.org/wiki/File:SolarSystemAbundances.png).

The reason for the zig-zag pattern is that, in general, elements with an even number of protons are more stable than those with an odd number, as discussed in [this post](https://astronomy.stackexchange.com/q/39561/5264).

Note that the $y$ axis is logarithmic, i.e. each tick mark denotes a ten-fold increase in abundance. The axis is arbitrarily scaled to $10^6$ silicon (Si) atoms, so the Si dot is at $\log A \equiv 6$. Oxygen, for instance, is at $y \simeq 7.15$, so it is $10^{7.15-6}\simeq14$ times more abundant than Si.

### Are we wrong?

The terminology used by astronomers may be considered wrong by other physicists and chemists. But why do we use the term at all? To distinguish some of the elements from other elements, based on their properties, I'd say. In astronomy, it very often makes sense to consider the elements of the Universe as divided in three:

* hydrogen, which is most of the mass and most of the atoms, and which provides most of the energy in stars,
* helium, which adds significantly to the mass, lowering the mass fraction of hydrogen, and
* all other elements, which together help cooling the gas (so it can condense), form dust grains (altering observed spectra), have a very roughly equal amount of protons and neutron (so $A\simeq2Z$), are formed in other processes than the Big Bang (except for a small part of the lithium), etc.

In physics, you may be more concerned with the material's ability to conduct electricity and hence consider sodium a metal until the pressure becomes too high, after which you call it a non-metal.

In chemistry, you may be more interested in the materials ability to form bonds, and hence do not consider arsenic and antimony metals.

So, I think you can say that there isn't a formal definition of the term "metal" — it depends on the context. From [Wikipedia](https://en.wikipedia.org/wiki/Metal):

> Around 95 of the 118 elements in the periodic table are metals (or are
> likely to be such). The number is inexact as the boundaries between
> metals, nonmetals, and metalloids fluctuate slightly due to a lack of
> universally accepted definitions of the categories involved

Apart from these practical reasons, there are also historical reasons, as discussed in [this answer](https://astronomy.stackexchange.com/a/12051/5264) by Rob Jeffries.


---

