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When I'm writing about atoms, often what I'm writing about applies to (monatomic) ions too. It's slightly annoying to add "(or ions)" after every mention of atoms, where a word that refers to both atoms and ions would be ideal. Does such a word exist?

There is "element", but often this is used to refer to multiple atoms of the same element, whereas "atom" and "ion" clearly refer to a single atom or ion.

An example: "An isotope is an atom (or ion) of the same element but with a different number of neutrons."

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    Physics or chemistry ... yes. As at any site in SE, showing your research is encouraged! – lbf May 13 '18 at 13:10
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    Atoms and ions are very different things. An Atom is the smallest particle of an element which can take part in a chemical reaction whereas the Ions are not capable of taking part in chemical reactions. Ions can, however, exist independently in a solution, whereas, Atoms may or may not be able to exist independently. Atoms/Ions. I think you are stuck with having to mention both. – Nigel J May 13 '18 at 13:16
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    A potential short form would be to write "(charged) atom of the same element". This is perfectly appropriate in academic writing in my field and would include both atoms and ions. – painfulenglish May 13 '18 at 14:37
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    I see you’ve already accepted an answer, and I’m not a frequenter on EL&U, but here are my suggestions: the International Bureau of Weights and Measures refers to both atoms and ions (in the SI Brochure) with the umbrella term elementary entities, where those entities are to be specified; also, with enough context, particles should suffice, in my opinion. – Chase Ryan Taylor May 13 '18 at 17:22
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    I'm voting to close this question as off-topic because this requires knowledge of Chemistry more than knowledge of English. – AndyT May 14 '18 at 13:11
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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).

  • As the OP now stands, your answer is a good fit and I have up-voted it. As it was first asked (in the first of its many edits) this answer would not have been correct. – Nigel J May 13 '18 at 17:59
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    Similar to ions are radicals. These are formed when input energy disturbs a covalent bond. They can be electrically neutral and are not necessarily ions. – Peter Wone May 14 '18 at 4:41
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    An alternative would be to use an introductory sentence like: "Unless specified otherwise, we will henceforth use the term 'atom' for both neutral and ionized atoms" or similar. – Dubu May 14 '18 at 16:55
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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.

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    @terdon 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. So says wikipedia – k1eran May 13 '18 at 13:05
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    @178865 an ion is any chemical moiety (moiety, by the way, is a good word here) with a charge. That can range in size from a single proton (H+) to huge, multi-atomic molecules. Calling them particles would suggest (to my mind, at least, although k1eran says otherwise) that you are referring to sub-atomic particles. – terdon May 13 '18 at 13:11
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    The word 'particle' does not mean just atoms and ions. It could refer to a quark, a lepton, a muon or a tau. To use it in reference only to chemically interacting species (atoms and ions) would not be correct. – Nigel J May 13 '18 at 13:11
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    even the '.photon' can be a particle – lbf May 13 '18 at 13:15
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    I see what you mean (and have removed my downvote) but I would strongly urge the OP not to use this term. Unless made very, very clear, the word particle will be taken to mean subatomic particle if discussing atoms. – terdon May 13 '18 at 13:51
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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.

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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.

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    So the word I'm looking for doesn't exist because it shouldn't exist? – 178865 May 13 '18 at 14:02
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    @178865 It would cause confusion if it did exist, so let's not invent it. – Nigel J May 13 '18 at 14:07
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    The dogs and apples comparison isn't very fair, you get the ion by taking out or adding an electron or proton (so the charge changes, in case of monatomic ions). A better comparison would be softdrinks before and after the sugar tax came in the UK, the only difference is the sugar content (sugar representing the charge in the atom/ion comparison). – JJJ May 13 '18 at 14:38
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    @JJJ At the atomic level, the removal, or addition, of an electron is of such significance that it transforms a neutral entity into a charged entity - and makes chemistry possible. Sweetening a drink just does not bear any comparison to the enormity of the atomic change, in my view. – Nigel J May 13 '18 at 14:42
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    @NigelJ as far as I know 'atomic particle' isn't regularly used. If atomic particles was commonly used for atom-size particles, it might be an answer, but as far as I see (and searched) it isn't, unfortunately. – JJJ May 13 '18 at 14:57
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In the vast majority of situations that apply to both atoms and ions, stating only atoms should be sufficient, as most people with a basic chemistry/physics background should understand it applies to both. (unless it also includes polyatomic ions)

In my opinion, the only time the distinction is warranted is for a case that applies only to ions and not neutral atoms.

Using terms like 'particle' or 'chemical species' are ok, but risk being too general and may harm the clarity of what you are trying to communicate.

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The word "species" and phrase "chemical species" are quite general and might serve your purpose. In chemistry, species can apply to atoms, ions, molecules, and radicals. The word "species" is used many times in the IUPAC Gold Book and chemical species is defined as:

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...

k1eran suggested "particle", which very general and is used by IUPAC Gold Book in the definition of both ion and atom:

ion: An atomic or molecular particle having a net electric charge.

atom: Smallest particle still characterizing a chemical element. It consists of a nucleus of a positive charge ( is the proton number and the elementary charge) carrying almost all its mass (more than 99.9%)and electrons determining its size.

Another phrase used throughout the IUPAC Gold Book that could apply is "molecular entity", which, according to the definition, includes atoms. You can see it used in the definition of "chemical species" above.

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.

  • Species is IMHO not a good fit: OP asks for a term that is the superset of monatomic ions and neutral atoms, whereas species specify subsets even of monatomic ions: Cu⁺, Cu²⁺, and Cu⁰ atoms are different species. – cbeleites May 15 '18 at 17:54
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If a statement applies to both atoms as well as monatomic ions (how about plasma?), chances are that it actually applies to the nucleus as such. If it doesn't apply to the nucleus, chances are that your statements do not extend to arbitrary levels of ionization.

So it may be more of a chemical than a linguistic question after all what you may best be writing here.

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