My level of chemistry knowledge is restricted to that which is given at high school level.

I was tought that an element is a substance that consists of only one type of atoms, meaning the number of protons is the same for every atom. Is this definition sound?

In case it is sound:

Since ozone gas only consists of oxygen atoms, would it be correct to say that ozone gas, as an entity, is an element?

Would it be more correct to say that ozone is an allotrope of the element oxygen?

Why is it called the table of elements, when it seems that we are actually listing the atoms and not the elements themselves?

  • $\begingroup$ Looks you're thinking too much about definitions ;) And it is one of few allotropes. $\endgroup$
    – Mithoron
    Sep 16, 2015 at 23:49
  • $\begingroup$ Well, it happens that I am interested in having the correct definitions so that I can make precise statements and be sure that I understand things correctly. $\endgroup$
    – Improve
    Sep 16, 2015 at 23:55
  • 2
    $\begingroup$ You may call ozone an elementary substance. (Not an element, that's for sure.) $\endgroup$ Sep 17, 2015 at 8:14
  • $\begingroup$ @IvanNeretin Then am I right in assuming that I can't refer to the substance oxygen gas as an element? I understand that this source goldbook.iupac.org/C01022.html claims that it is optional. $\endgroup$
    – Improve
    Sep 17, 2015 at 12:37
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    $\begingroup$ Wow! That's an interesting analogy, though not without certain limitations. We may think of ordinary tin as the canonical allotrope and gray tin as the other one. Now, there are cases when we have a bunch of non-canonical allotropes (like carbon with diamond, fullerenes, nanotubes, lonsdaleite and who knows what else), but then again, one might think of $\sqrt[n]x$ with its complex values... Actually, it's phosphorus where the scheme fails; it has no undisputed canonical allotrope. See, white phosphorus is "canonical" in terms of $\Delta G$, and red phosphorus is the easiest to come by, but... $\endgroup$ Sep 17, 2015 at 21:45

2 Answers 2


It would be more correct to describe ozone as an allotrope of oxygen. Dioxygen (about 1 in five of the molecules making up air) is another allotrope.

The periodic table describes the atoms that make up elements as this is the most fundamental property. Many other elements, though, exist in multiple allotropes. Carbon, for example, has several: graphite, diamond, buckminsterfullerene and quite a few others.

All allotropes consist of only one type of atom but the bulk physical properties depend on how the atoms are joined up (flat plates of carbon hexagons with multiple bonding in graphite, a three-dimensional network of singly-bonded tetrahedral carbons in diamond).

We don't normally add too much about allotropes in a periodic table as it would make the table a little big, although some include limited information about how many there are. And, usually, only one is thermodynamically stable under normal conditions (so the physical properties will be about that one).


O3 is a allotropic form of oxygen which is a element and also O3 is made up by same atom so, we can called O3 a element


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