Is there a single word for both "atom" and "ion"?

Solution 1:

My suggestion is that at the first mention of the word atom, you add a qualifier like neutral and/or ionized. For example, you can use one of the following constructions:

...neutral or ionized atoms...
...both neutral and ionized atoms...
...atoms, whether neutral or ionized, ...
...atoms, both neutral and ionized, ...

After that, just use the word atom. The reader will know that you mean both neutral and ionized species.

In parts of the text where it is important to distinguish electrically neutral species from electrically charged ones, use the expressions neutral atoms and ionized atoms.

Discussion

In many (though not all) contexts, the word atom already denotes both the ionized and the non-ionized species. This is why when it is important to emphasize the status with respect to electric neutrality, we speak of neutral atoms and ionized atoms.

I agree that the default reading of the word atom is one where it is electrically neutral. But I disagree that electric neutrality is part of the very definition of that word.

(As others have emphasized already, an ion is not necessarily an ionized atom; an ionized molecule is also an ion.)

Wikipedia is of course not itself a reputable source, but consider the second sentence in its article on atoms (boldfaced emphasis mine):

Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.

Here are some examples of similar usage from published sources (boldfaced emhases mine):

A rare-earth or actinide atom, whether neutral or ionized, very often possesses several electrons outside closed shells... (source)
identified by the spectra of their neutral or ionized atoms (source)
Tables of Spectral Lines of Neutral and Ionized Atoms (source)
the fact that both neutral and ionized atoms are emitting (source)

In all of the examples, it is clear that the authors say that atoms can be both neutral and ionized, which wouldn't make much sense if atoms were neutral by definition; see below for more on that.

Zumdahl's Chemistry

To be sure, there seem to be examples where electric neutrality seems to be part of the definition of the word atom. For example, in the university-level introductory-chemistry textbook by Zumdahl, we find the following:

Since atoms were known to be electrically neutral... (p. 46)

But Zumdahl is not consistent in this usage. Consider his definition of an ion:

An ion is an atom or group of atoms that has a net positive or negative charge. (Zumdahl, p. 56)

This definition would make no sense if atoms had to be electrically neutral by definition; if they did, it would not be possible for an atom to have a net positive or negative charge. To drive this point home, note how strange the above definition becomes if atom is qualified by neutral:

^?An ion is a neutral atom or group of atoms that has a net positive or negative charge.

'How can a neutral atom have a net charge??' is what we immediately want to ask. In order for the sentence to make sense, we would want to rephrase, e.g. as follows:

An ion is a neutral atom or group of atoms that has acquired a net positive or negative charge.

Now it's OK, because a thing can change group membership after something happens to it; compare with an odd number is an even number to which we added 1.

Similarly, if it were really the case that atoms were electrically neutral by definition, it would be a bit strange to talk about neutral atoms—after all, what other kind could there be? And yet, we find the following in Zumdahl (boldfaced emphases mine):

In a neutral atom of each element, how many electrons are present? (p. 71)
...the symbol and mass number of a neutral atom whose total mass of its electrons is... (p. 74)
Note that the first electron is removed from a neutral atom (Al), ... (p. 309)
The number of valence electrons on the free neutral atom... (p. 364)

Zumdahl is definitely not alone in using the expression neutral atom. It is in fact widely used in scientific literature, as a search in google books can easily verify (see here).

Solution 2:

particle

I admit particle is a pretty overloaded term, which indeed is very often applied to subatomic world, but I don't think that precludes its use for molecular/atomic level entities.

The following BBC example for Chemistry students below seems to say same...

Ions are electrically charged particles formed when atoms lose or gain electrons. They have the same electronic structures as noble gases.
— BBC

And from Wikipedia:

In the physical sciences, a particle (or corpuscule in older texts) is a small localized object to which can be ascribed several physical or chemical properties such as volume , density or mass. They vary greatly in size or quantity, from subatomic particles like the electron, to microscopic particles like atoms and molecules, to macroscopic particles like powders and other granular materials.

[...]

The term 'particle' is rather general in meaning, and is refined as needed by various scientific fields.

Note:

Above answer was written before the OP supplied the sample sentence. In hindsight I think he was not really asking about ions/atoms but rather needed the word form. e.g. Isotope are forms of the same element ... I'm leaving it as I think discussion on whether it can be use at an atomic/molecular level or better reserved for sub-atomic level only is interesting.

Solution 3:

Monatomic ions are a type of atom.

You seem to be asking for a word that encompasses both monatomic ions and neutral atoms. That word is atom.

Solution 4:

Charged ions and stable atoms are different things physically and chemically.

They require to be identified separately.

We are not, here, comparing apples and oranges or cats and dogs. We are referring, as it were, to dogs and apples. So they have to be identified as separate items.

An atom can be an ion, but not all ions are atoms. The difference between an atom and an ion has to do with net electrical charge.

An ion is a particle or collection of particles with a net positive or negative charge.

An atom is the basic unit of an element. The identity of an element is determined by the number of positively charged protons in the atom’s nucleus. A stable atom contains the same number of electrons as protons and no net charge.

Science Notes

NOTE : This answer was to a previous edit of the OP. I have up-voted @linguisticturn because I believe that it is now a good fit for the question as it currently stands.