What is an atom?


the smallest unit of any chemical element, consisting of a positive nucleus surrounded by negative electrons.

Cambridge Dictionary

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

IUPAC Gold Book

There is something terribly wrong with these definitions.

According to IUPAC, an atom must be neutral: protons and electrons have equal and opposite charges, so Z protons and Z electrons have a net charge of 0.

The Cambridge definition differs, permitting any nucleus with at least one electron to be called an atom. Similar definitions appear at dictionary.com, lexico.com, and macmillandictionary.com. The Cambridge definition is representative of the latitude of common definitions of ‘atom’, and I use it for no other reason.

\begin{array}{r | c c c c} \textbf{number of electrons} & 0 & 0 < x < Z & Z & > Z \\ \textbf{state} & \text{positive nucleus} & \text{positive} & \text{neutral} & \text{negative} \\ \textbf{example} & \text{He}^{2+} & \text{He}^+ & \text{He} &  \text{He}^- \\ \\ \textbf{IUPAC} & & & \text{'atom'} & \\ \textbf{Cambridge} & & \text{'atom'} & \text{'atom'} & \text{'atom'} \end{array}

Some might dismiss the distinction. After all, chemists know the difference? If the layman’s definition isn’t perfect, it’s not going to cause any problems, right?

Alas not. The truth is even chemists’ use of ‘atom’ is confused…

The Neutral Atom

In the US and the UK, understanding an atom in terms of the numbers of protons, neutrons, and electrons is an expectation typically placed on teenagers. ‘Atom’, ‘proton’, ‘neutron’, and ‘electron’ will be taught much earlier but not in a quantitative setting.

Atoms will be introduced as neutral, and questions will rely on that understanding…

An atom contains 6 protons. How many electrons does it have?

‘Correct’ answer: 6

So far, no problem. It’s perfectly reasonable to ask a student to remember and apply a technical definition that differs from the popular one.

However, the use of ‘atom’ rapidly departs from that rigor.

The Non-neutral Atom

Ionic Compounds

This is sodium chloride:


Composed of sodium (\text{Na}^+) and chloride (\text{Cl}^-) ions, sodium chloride–also known as table salt–is often introduced as the prototypical ionic solid.

The synthesis of sodium chloride from sodium and chloride is a relatively common subject for demonstrating balancing a chemical reaction equation:

\begin{aligned} \text{Na} + \text{Cl}_2 &\longrightarrow \text{NaCl} \\ \textbf{2} \text{Na} + \text{Cl}_2 &\longrightarrow \textbf{2}\text{NaCl} \end{aligned}

However, students are repeatedly told to balance atoms, not ions, as if ‘atom’ can mean ‘atom-like’. It is disruptive to learning to so swiftly break the rules that have just been drummed in.

Relative Formula Masses

To calculate the relative formula mass of \text{NaCl}, students are taught to find the relative atomic masses of sodium and chlorine from the periodic table:

\begin{array} {|c|} 11 \\ \text{\Huge Na} \\ \text{sodium} \\ 22.990 \end{array}\begin{array} {|c|} 17 \\ \text{\Huge Cl} \\ \text{chlorine} \\ 35.45 \end{array}

\begin{aligned} RFM(\text{NaCl}) &= 22.990 + 35.45 \\ &= 58.44 \end{aligned}

This explanation bypasses the fact that sodium chloride is ionic. On the surface, this is not a problem because an electron has a relative formula mass of approximately 0.0005.

\text{mass of an electron} = 9.10938 \times 10^{-31} \text{ kg}

\begin{aligned} \frac{1}{12}^{th} \text{ mass of a carbon-12 atom} &= \frac{0.001 \text{ kg}}{\text{Avogadro's number}} \\ &= \frac{0.001 \text{ kg}}{6.02214 \times 10^{23}} \\ &= 1.66054 \times 10^{-27} \text{ kg} \end{aligned}

\begin{aligned} RFM(\text{e}^-) &= \frac{\text{mass of an electron}}{\frac{1}{12}^{th} \text{ mass of a carbon-12 atom}} \\ &= 5.48580 \times 10^{-4} \\ &\approx 0.0005 \end{aligned}

However, this means that the relative formula masses of sodium and the sodium ion actually differ by 0.0005.

RFM(\text{Na}) = 22.990

\begin{aligned} RFM(\text{Na}^+) &= 22.990 - 0.0005 \\ &= 22.9895 \end{aligned}

Again, a meaningless difference, particularly given that the missing mass from the missing electron in \text{Na}^+ is compensated for by the extra electron in \text{Cl}^-.

\begin{aligned} RFM(\text{NaCl}) &= RFM(\text{Na}^+) + RFM(\text{Cl}^-) \\ &= RFM(\text{Na}) - RFM(\text{e}^-) + RFM(\text{e}^-) + RFM(\text{Cl}) \\ &= RFM(\text{Na}) + RFM(\text{Cl}) \end{aligned}

The issue is that this is one more case where ions are treated like atoms, compounding the confusion in students over what an atom actually is.

Resolving the Conflict

This is the current landscape:

\begin{array}{r|c c} \textbf{entity} & \text{Na} & \text{Na}^+ \\ \textbf{IUPAC} & \text{atom (neutral)} & \text{ion (charged)} \\ \textbf{usage} & \text{atom} & \text{ion/atom} \end{array}

The word ‘atom’ is applied to entities that do not fit the IUPAC definition.

There are three solutions I propose…

Solution 1 – A new word for atom or ion

A new word that means “a nucleus with at least one bound electron” would empower chemists to refer to atoms, ions, or mono-nuclear entities that could be either.

My expertise in etymology and lexicography is limited, so my tentative suggestion is: “monon” (mon-on).


smallest particle still characterizing a chemical element. It consists of a nucleus containing Z protons with or without neutrons, and one or more bound electrons.

Nathan March

Solution 2 – Adopting the common usage of ‘atom’

Instead of using ‘monon’, ‘atom’ could be used for any mononuclear entity with electrons. The phrase ‘neutral atom’ could then apply when neutrality is required, or another word could be coined.

Solution 3 – Using ‘atom’ rigorously

The last option would be to simply use ‘atom’ according to its definition, and never for an ‘ion’. The constituents of ionic compounds would always be referred to as ions, and balancing of chemical equations would be by balancing nuclei and charge.


\begin{array}{r | c c c} \textbf{number of electrons} & 0 < x < Z & Z & > Z \\ \textbf{state} & \text{positive} & \text{neutral} & \text{negative} \\ \textbf{example} & \text{He}^+ & \text{He} & \text{He}^- \\ \\ \textbf{IUPAC} & \text{'ion'}& \text{'atom'} & \text{'ion'} \\ \textbf{common usage} & \text{'atom'/'ion'} & \text{'atom'} & \text{'atom'/'ion'} \\ \\ \textbf{solution 1} & \text{'monon'/'ion'} & \text{'neutral monon'/'atom'} & \text{'monon'/'ion'} \\ \textbf{solution 2} & \text{'atom'/'ion'} & \text{'neutral atom'} & \text{'atom'/'ion'} \\ \textbf{solution 3} & \text{'ion'} & \text{'atom'} & \text{'ion'}\end{array}

The Advantages of the Monon

Solution 3: Applying ‘atom’ according to the IUPAC definition would apply considerable pressure to educators to update their materials and be explicit about the charge of a mononuclear entity in all cases. It is unlikely to occur as it would highlight past materials as ‘incorrect’ and create a discrepancy between past and future materials.

Solution 2: Adopting the common usage of ‘atom’ might work and, in fact, ‘neutral atom’ is a phrase that occurs embarrassingly often (science-direct.com, khanacademy.org, nature.com), despite it being technically a tautology–no doubt adopted because of the inevitable clash between the common usage and the technical definition.

Solution 1: ‘monon’ solves the conflict by the path of least resistance, letting the ‘atom’ recede over time to its technical definition, and providing no pressure to revise past works.

Another advantage of ‘monon’ is that it can be extended to multiples of itself using the common numerical prefixes.

\begin{array}{r | l} \textbf{monon} & \text{single atom or mononuclear ion} \\ \textbf{duon} & \text{two monons} \\ \textbf{trion} & \text{three monons} \\ \textbf{tetron} & \text{four monons} \\ \textbf{...} & \\ \textbf{polyon} & \text{many monons}\end{array}

This allows new families of chemicals to be talked about with ease.

\begin{array}{r | l} \textbf{family} & \textbf{examples} \\ \text{monons} & \text{H}^+, \text{He}, \text{Cl}^- \\ \text{duons} & \text{H}_2, \text{HBr}, \text{NaCl} \\ \text{trions} & \text{MgCl}_2, \text{O}_3, {\text{N}_3}^- \\ \text{tetrons} & \text{AlCl}_3, \text{BH}_3, {\text{NO}_3}^- \end{array}

Would you like to adopt this or do you have a better suggestion? Let me know in the comments.

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