A single word for "atoms and molecules"?

My professor often expresses frustration because, inconveniently, there is no word that groups together "atoms" and "molecules". "Microscopic particles" is totally insufficient.

I now find myself experiencing this frustration.

Chemists/Physicists often find themselves saying, especially in Statistical Mechanics, e.g. "Of a set of N atoms and molecules, we select one at random." If we say "particle", it excludes molecules. If we say "molecules", it excludes atoms.

As a bonus, is there perhaps such a word in another language? e.g. German or French?

Solution 1:

The IUPAC Goldbook defines two terms that are generic to atoms and molecules.

Molecular entity:

Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer etc., identifiable as a separately distinguishable entity. Molecular entity is used in this Compendium as a general term for singular entities, irrespective of their nature, while chemical species stands for sets or ensembles of molecular entities. Note that the name of a compound may refer to the respective molecular entity or to the chemical species, e.g. methane, may mean a single molecule of CH4 (molecular entity) or a molar amount, specified or not (chemical species), participating in a reaction. The degree of precision necessary to describe a molecular entity depends on the context. For example 'hydrogen molecule' is an adequate definition of a certain molecular entity for some purposes, whereas for others it is necessary to distinguish the electronic state and/or vibrational state and/or nuclear spin, etc. of the hydrogen molecule.

and

chemical species:

An ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on the time scale of the experiment. The term is applied equally to a set of chemically identical atomic or molecular structural units in a solid array. For example, two conformational isomers may be interconverted sufficiently slowly to be detectable by separate NMR spectra and hence to be considered to be separate chemical species on a time scale governed by the radiofrequency of the spectrometer used. On the other hand, in a slow chemical reaction the same mixture of conformers may behave as a single chemical species, i.e. there is virtually complete equilibrium population of the total set of molecular energy levels belonging to the two conformers. Except where the context requires otherwise, the term is taken to refer to a set of molecular entities containing isotopes in their natural abundance. The wording of the definition given in the first paragraph is intended to embrace both cases such as graphite, sodium chloride or a surface oxide, where the basic structural units may not be capable of isolated existence, as well as those cases where they are. In common chemical usage generic and specific chemical names (such as radical or hydroxide ion) or chemical formulae refer either to a chemical species or to a molecular entity.

Solution 2:

The word does not yet exist. It is the job of the people feeling the lack of the word to coin it. The OP implicitly admits this in two comments:

He's [my professor's] been looking for a word for years as it is.

I think it may be time to call a forum of scientists to make a new word.

Physicists have been coining names for particles for decades; some of the names were derived linguistically (baryon, neutrino), and some were pure whimsy (quark, axion).

Based on the example the OP gave:

Of a set of N atoms and molecules, we select one at random.

I suggest the acronym NAMs. There is a precedent for an acronym: WIMPS (Weakly Interacting Massive Particles). Now it is up to the OP to gain the towering reputation or to write the critical paper that will gain acceptance for this or whatever word or acronym the OP decides is better. :)