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Name:  Jenny
Status: student
Grade: 9-12
Country: N/A
Date: 5/9/2005

When naming chemical compounds, what happens if there is a semi-metal and a non-metal? Is the semi-metal treated as a metal or a non-metal? For example, is S02 named silicon oxide or silicon dioxide? What happens with a metal and a semi-metal?


Before I answer this question, I would ask you to remember that nomenclature rules are nothing more than that - agreed upon patterns. There is nothing fundamental about naming inorganic binary compounds.

Now, as to your question. It is best to make the distinction between metals and non-metals (rather than metals, metalloids and non-metals). If you look at a periodic table, you can usually find a "staircase" that starts at aluminum, down to gallium, right to germanium, down to tin, right to antimony, etc. Everything to the left of this staircase (including the elements I mentioned) should be considered metals, and everything to the right non-metals. As such, if you have a metal + non-metal then we name the compound "metal + 'space' + root of non-metal+'ide' ". So AlCl3 would be named aluminum chloride. If, on the other hand, we have a non-metal + non-metal (remembering that we do not make a distinction between metalloids and non-metals), then we name the compound using prefixes in the pattern: "prefix+name of first element + 'space' + prefix+root of second element+'ide' ". So SiO2 would be called silicon dioxide (remembering that we do not use "mono" if it is up front).

Greg (Roberto Gregorius)

Hi Jenny,

A great question!

Names are funny things. You go by "Jenny," but "Jennifer" is probably also correct. In addition you have a last name and possibly also one or more middle names. There are various names that you can be called correctly. Sometimes it is the same with chemicals as well. But other times, only one will do. However, unless you tell me your name I may or may not be able to guess it correctly. Now, we have a way to guess the right name for a binary compound. Our rule of thumb is; if it is made of a metal and a nonmetal, it is an ionic compound and we name it using the ionic compound rules. If it is made of two nonmetals, we name it using the covalent compound rules. However, the rules do not cover semimetals (metalloids).

This is because if a compound is made of a semimetal and a nonmetal, it may or may not be an "ionic compound." In the case of SiO2, the answer is "not." This means that SiO2 is not made of O ions with a 2- charge and Si ions with a +4 charge. Rather, Si is bonded to O covalently in the crystal lattice; electrons are shared between atoms, not transferred completely from one to the other.

As far as names go, SiO2 is named silicon dioxide, but since this is the only uncharged oxide of silicon you might also see it called silicon oxide in some references.

However, as you go farther away from O in the periodic table the electronegativity difference is greater and the oxides become ionic compounds. So for example According to "Inorganic Chemistry" by Shriver and Atkins (3rd ed., W.H. Freeman & Co., New York, 1999, p.387), As2O3 would be named arsenic (III) oxide (because As is a +3 ion and there is more than one possible charge state for an As ion). The most ambiguous case I can think of would be P4O10, which "is produced by the complete combustion of phosphorus" (Shriver and Atkins). P is close to O on the periodic table, but S&A say that this should be named phosphorus (V) oxide, implying that this is an ionic compound. To me, this name is a lot better than tetraphosphorus decoxide...but is it obvious that P4O10 should be named as if it were ionic? Not to most chemistry students (or even their teachers!).

I guess the answer is that, it depends. Some nonmetal/semimetal compounds should be named according to the Stock system (III, V, etc) and others should use prefixes to indicate the stoichiometry. Without knowing whether the compound is ionic or not, it may not be possible to give the name correctly.

Hope this helps,
Dr. Topper

I think you mean SiO2, not "SO2". The correct name is silicon dioxide. In general, the element to the "left" and "toward the bottom" of the periodic table (excluding hydrogen, sometimes) comes first in the name of a compound. But there are exceptions to this "general" rule where common names are involved.

Vince Calder


I am not sure that inorganic compounds have as good a formalized naming system as organic compounds do. A consistent, formal naming system might be something less-than-handy like silicon (IV) oxide for SiO2. Most people just say silicon dioxide, counting the oxygens. Then silicon monoxide, SiO, if you accept it as a stable compound, would be silicon (II) oxide. If we wanted we could call them "silicic" and "silicous" oxides (there is "silicic acid"), but we do not feel like doing that.

I dislike that Ta2O5 is commonly called "tantalum pentoxide". It requires knowing the favored oxidation numbers of Ta to mentally derive the "2". "Tantalum (V) oxide" requires knowing only that the oxidation number of oxygen is -2, which is so much more widely used and known.

Chromium makes Cr2O3, a stable hard greenish crystal or ceramic vaguely like sapphire (Al2O3). It also makes CrO3, a low-melting, low-boiling, water-soluble acid with weak crystals with a deep red/purple/brown color. It is a strong oxidizer, but it is not called chromium peroxide. It is called Chromic acid. So you tell me, which of these two is to be called "Chromium Trioxide"?

I do not think there is any consistent distinction in the naming of oxides of elements metal, semi-metal, or non-metal. It seems to me that "ic" and "ous" are merely convenient language assets that get applied wherever linguistically profitable. Similarly with mono-, di-, tri-, sesqui-, sub-, per-, and others.

Titanium is a metal, and TiO2 is called titanium dioxide not titanic oxide. Certainly there are lower oxides such as TiO or Ti2O3 to accept the relative-oxidation description "titanous oxide". But "titanic and titanous" are never used. I think it is because they sound weird to us. Not exactly a scientific naming system.

CuCl2 is "cupric chloride". CuCl is "cuprous chloride". Copper is a metal element. NO is "nitric oxide". N2O is "nitrous oxide". NO2 is "nitrogen dioxide" or, being always in equilibrium with N2O4, "nitrogen tetroxide". (hmmph.) Nitrogen is a nonmetal element. Sb2O5 is "stibnic oxide", and Sb2O3 is "stibnous oxide". Likewise arsenic and arsenious. I suppose Sb and As are considered semi-metals, along with Selenium and Tellurium. Bismuth is a problem. How would you like to be saying "Bismuthous" or "Bismuthic"? Maybe they should use "bismous"...

Every now and then the prefix "sesqui-" gets used to denote an oxide in between two small-integer oxides. Fe2O3 is "Ferric Oxide", FeO or Fe2O2 is "ferrous oxide", so Fe3O4 is left out, and perhaps called "sesquioxide'. Notice that in Fe3O4 the iron's average oxidation number is 3.666..., not even an integer. You can think of it as a crystal structure allowing a stable integer-ratio mixture of oxides: 2 Fe(III) + 1 Fe(II) oxides. It is apparent that we have a ways to go before we get to a rolling-off-the-tongue naming system that could cover that.

In that kind of combinatorial chaos, all kinds of common names have thrived, and literature falls back on chemical formulas and mineral names when it is necessary to be more specific. Mineral names are used to denote not just the formula but also a specific crystal structure.

Jim Swenson

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