What are compounds anyway?

According to Oxford English Dictionary, “compound” (in chemistry) is

a substance formed from two or more elements chemically united in fixed proportions.

(1)

I quite like this definition. There are four statements in it:

• compound is a substance (therefore, it is macroscopic);
• compound contains at least two (different) elements;
• these elements are “chemically united”, i.e. chemically bound;
• they are bound in fixed proportions.

The definition of compound from Carey’s Organic Chemistry [1] is totally different:

An assembly of two or more atoms with properties different from the individual atoms.

(2)

So compound (2) appears to be microscopic. Nothing prevents it to be composed of the atoms of the same element: the properies of dichlorine (Cl₂) are very different from properties of atomic chlorine radical (Cl·). The individual atoms as well as their “assemblies” are examples of molecular entities.

Compare that with IUPAC’s much more vague description from Principles of Chemical Nomenclature [2]:

Compounds are composed of atoms of the same or of more than one kind of element in some form of chemical combination.

It is not clear what compounds are at all. If they are “composed of atoms of the same kind of element”, they are not different from elementary substances.

But wait. On the same page we read:

Throughout this discussion, we have been considering pure substances, i.e. substances composed of a single material, whether element or compound. A compound may be molecular or ionic, or both. A compound is a single chemical substance.

(3)

So, according to (3):

• a compound is a single chemical substance (not defined in the Principles)
• pure substance is composed of a single material (also left undefined)
• compounds are not elementary substances (here called “elements”)
• a compound is a type of single material
• an elementary substance is another type of a single material

Are you confused? I am for sure.

Let’s stick to the (pure) substance being macroscopic. If it is composed of compound, doesn’t it mean that compound is smaller than substance? And if it is, is compound microscopic, macroscopic or somewhere in between?

The indirect answer comes from the Red Book’s definition of isotopically substituted compounds and isotopically labelled compounds [3]:

An isotopically substituted compound has a composition such that all the molecules of the compound have only the indicated nuclide(s) at each designated position.

Thus compounds, indeed, are macroscopic and consist of molecules.

An isotopically labelled compound may be considered formally as a mixture of an isotopically unmodified compound and one or more analogous isotopically substituted compounds.

So compounds can be mixtures of more than one compound! Clearly macroscopic. Can we say that, at least, some compounds are not “pure substances” then? I think we can and we should.

In any case, “pure substance” is an abstraction — a useful one but abstraction nonetheless. Even so-called “absolute alcohol” contains compounds other than ethanol. Even the purest water contains molecular entities other than water molecules, for example ions OH^– and H₃O⁺.

I prefer to think of, and will refer to, “compound” as macroscopic, built of microscopic units that we call “molecular entities” consisting of at least two different elements. Therefore, compounds do not include elementary substances. Now, we can distinguish between the names of compounds — say, “water” — from the names of their constituents — say, “molecule of water”. If we treat chemical formulae as names, we can say that empirical formula H₂O is another name for the compound “water” and structural formula H–O–H is another name for the molecule of water.

1. Carey, F.A. and Giuliano, R.M. Organic Chemistry, 9^th Ed., McGraw-Hill, 2014, G-5.
2. Leigh, G.J., Favre, H.A. and Metanomski, W.V. Principles of Chemical Nomenclature: A Guide to IUPAC Recommendations. Blackwell Science, 1998, p. 7.
3. Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005, p. 64.