# Elements which do not form oxides

I had thought that this question would be easy to answer but my searching has been unsuccessful in finding a clear answer.

For the metals, I expect that they all form oxides. Some very readily, e.g. sodium and magnesium, and some reluctantly, e.g. gold and platinum, but I cannot find a metal which does not form an oxide.

The metalloids also all appear to form oxides.

The noble gases are obvious candidates for exceptions but xenon has a known oxide. I think that it is expected for radon but I don't know whether that is confirmed. A fluoride is known for krypton but no oxide yet. No oxide is known or expected (?) for any of the lighter noble gases.

This leaves the other non-metal elements which are sufficiently few to check easily. I find no exception except for the possible pedantic case of fluorine. It forms a binary compound with oxygen but since it is more electronegative the compound is called oxygen fluoride rather than fluorine oxide.

Some elements are too radioactive to study e.g. francium and astatine but I expect that both would form oxides if we could manage to study them.

Edit: a good point that came up in the comments is oxygen itself. A pedantic exception together with fluorine.

Is that correct?

• @andselisk I may have misunderstood. Oxygen is a good answer. A subtle exception alongside fluorine. – badjohn Nov 23 '17 at 12:09
• chemistry.stackexchange.com/a/64699/9961 – Mithoron Nov 23 '17 at 15:57
• Eh, this bothers me enough that I have to ask: Should the subject read "Elements that" instead of "Elements which"? (Not a native speaker, it just looks wrong somehow) – JollyJoker Nov 23 '17 at 16:04
• @JollyJoker A good point. It's a question for a rather different stack exchange but this Oxford Dictionaries article seems to be on your side: en.oxforddictionaries.com/usage/that-or-which. It says: "This common British construction is not strictly incorrect in American English, but it is generally avoided, especially in formal writing" which may explain it. I am British, are you more familiar with US English? – badjohn Nov 23 '17 at 16:12
• @Mithoron Thank you. That's interesting though my question was not primarily about the chemistry of noble gases but more whether I had missed anything else. – badjohn Nov 23 '17 at 16:40

The first thing that came to mind is oxygen itself, as the term "oxide" suggests one other element in its chemical formula. Merriam-Webster suggests a more strict version: "a binary compound of oxygen with a more electropositive element or group". So technically pure oxygen is not an oxide.

Putting oxygen aside, helium $\ce{He}$, neon $\ce{Ne}$ and argon $\ce{Ar}$ are probably the only three conventional elements that come to mind are not yet proven to form oxides. I find it rather tricky to back up this statement properly as the lack of knowledge about an object doesn't prove its non-existence; ideally one might want to do a stability simulation at extreme pressures and temperatures to prove the existence of $\ce{(He|Ne|Ar)O_x}$ or lack thereof. There are algorithms like USPEX that are capable of that – by the way, with the help of USPEX a family of krypton oxides has been recently predicted.

• Thanks. I also considered fluorine as an exception since as you quote "a more electropositive element" whereas fluorine is more electronegative. The rule I knew was that in binary, inorganic compounds, the more electronegative element is named second. $\ce{O}\ce{F}_2$ rather than $\ce{F}_2\ce{O}$. – badjohn Nov 23 '17 at 13:40
• Yes, oxygen can by definition not form oxides. To the best of my knowledge, no compounds of helium or neon are known to date and only $\ce{HArF}$ is known for argon, but that could be dated already. – Jan Nov 23 '17 at 14:24
• " lack of knowledge about an object doesn't prove its non-existence" - of course, but I am happy with answers of the form: "no oxide of X is known". So, krypton might be in the category "no oxide known but that might change" whereas neon is "no oxide known and none expected either". – badjohn Nov 23 '17 at 14:34

So, combining all of the responses.

All metals and metalloids have known oxides.

Among the noble gases, xenon and radon have oxides, one has been predicted but not observed for krypton, and one is not expected for helium, neon, or argon.

Among the other non-metals, all have oxides except the technical exceptions of oxygen itself and fluorine; as andselisk says, an oxide is "a binary compound of oxygen with a more electropositive element or group". A binary compound of oxygen and fluorine is known but it is oxygen fluoride rather than fluorine oxide.

• Would that include astatine and francium? – Oscar Lanzi Jan 5 '20 at 15:04
• A good point. I expect that if we could move quickly enough, we could create francium oxide and astatine oxide. The problem is the physics, I doubt that there is any chemical obstacle to these compounds. – badjohn Jan 5 '20 at 17:09
• The intense radioactivity of all isotopes of astatine and francium would prevent collecting any macroscopic samples of any oxides, for the heat generated by radioactive decay would vaporize such samples. Wikipedia reports several empirically known compounds with astatine-oxygen bonds in solution. – Oscar Lanzi Mar 3 '20 at 1:17