I was told that an atom's atomic number is defined as follows:

The number of electrons or protons present in a neutral atom is called atomic number. It is represented by Z.

What does neutral mean here? Why isn't it just "..present in an atom..."?

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    $\begingroup$ "Neutral atom" is close to nonsense, at least superfluous. An atom is neutral by definition, if not, it is called an ion. Its another instance of white swan, green grass or black soot. $\endgroup$
    – Georg
    Jul 17, 2012 at 9:42
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    $\begingroup$ @Georg There are certainly swans, that are not white ;) $\endgroup$ Jul 20, 2014 at 5:12
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    $\begingroup$ It's a bad definition of atomic number. The electron count is only equal to the atomic number in a neutral atom but the term is meaningless outside that context as there are better definitions of atomic number: just count the protons! $\endgroup$
    – matt_black
    Jan 15, 2016 at 23:03
  • $\begingroup$ @Georg: So table salt ain't made of atoms? $\endgroup$
    – Vikki
    Dec 15, 2019 at 3:39

5 Answers 5


Electrons and protons are charged particles. The electrons have negative charge, while protons have positive charge. A neutral atom is an atom where the charges of the electrons and the protons balance. Luckily, one electron has the same charge (with opposite sign) as a proton.

Example: Carbon has 6 protons. The neutral Carbon atom has 6 electrons. The atomic number is 6 since there are 6 protons.


Here, a "neutral atom" is simply an atom that has no charge.

See, an atom consists of protons, neutrons, and electrons. Protons are positively charged, electrons are negatively charged (with the same magnitude of charge per particle as a proton). Neutrons have no charge.

Now, in a "neutral atom", the number of protons must be equal to the number of electrons, otherwise it would not be neutral.

Basically, the definition is saying that "in a neutral atom, the atomic number is equal to both the number of protons , and the number of electrons, because, well, they both are the same."

Compare this with an ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge

Here, the atomic number cannot be equal to both--because they are unequal. In this case, a better definition would be:

The number of protons present in an atom is it's atomic number. It is represented by $Z$.


I totally agree with George that "neutral atom" is close to nonsense. Indeed, IUPAC recommends the following definition for atom:


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.

As one can see, for an atom the proton number, also known as atomic number, which is the number of protons in the atomic nucleus, is the same as the number of electrons. Consequently, any atom is (electrically) neutral by definition, as George mentioned in his comment.

  • $\begingroup$ Because the phrase "hydrogen-like atoms" for the series, H, He+, Li2+, Be3+, etc. is often used, I don't think "neutral atom" is nonsense. $\endgroup$
    – DavePhD
    Feb 8, 2016 at 21:13

Because sometimes scientists use the term "atom" when it is not neutral.

This is most often in the phrase "hydrogen like atoms":

A hydrogen like atom consists of one nucleus of charge Ze and a single electron of charge -e.

See also:

Correlation in Helium-Like Atoms (2-electron atoms regardless of charge)

The Spectra of some Lithium-like and Sodium-like Atoms (3 and 11-electron atoms)

Quadruply excited beryllium-like atoms – a semiclassical model (4-electron atoms)

Fine-structure studies of the ground state of boron-like atoms (5-electron atoms)

Relativistic corrections to the ground state energies of the carbon-like atoms (6-electron atoms)



Neutral atom is an atom with no net charge on it like it is not Na+ but Na or Cl- but Cl. The person has not referred to atom but neutral atom probably because charged atoms/ions have extra electron (as Cl-) or lesser number of electrons (like Na+), violating the concept that atomic number is the number of protons/ electrons present in a neutral atom.

  • $\begingroup$ Atomic number is the number of protons present in an atom, neutral or not. No rule is violated. $\endgroup$
    – bon
    Feb 8, 2016 at 22:06
  • $\begingroup$ Well according to the afore mentioned definition when it isor electron it does violate the concept. $\endgroup$
    – Tyto alba
    Feb 9, 2016 at 4:38

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