I've read this excerpt from Wikipedia countless times, but I'm still confused:

The Avogadro number is the approximate number of nucleons (protons or neutrons) in one gram of ordinary matter.

Shouldn't that be "in one mole"?


1 Answer 1


The quote you have used is there just behind the Avogadro constant([$\pu{mol-1}$])/Avogadro number(unitless) definition. Many true statements can be misinterpreted, if quoted or considered out of their context.

Notice the approximate. It is not the definition.

  • $\pu{1 mol}$ is by definition the amount of matter consisting of exactly $N_\mathrm{A} = \pu{6.02214076E23}$ particles(or generally any formal constituent objects).
  • The molar mass of nucleons ( protons or neutrons) is approximately $M_\text{nucleon} \approx \pu{1 g/mol}$.
  • Practically all the mass of ordinary matter is due its nucleons.
  • Therefore, $\pu{1 g}$ of ordinary matter contains approximately $N_\mathrm{A}$ nucleons, as it contains approximately $\pu{1 mol}$ of them.

Imagine this:

  • I say that the current definition of kilogram is based on the fixed value of the Planck constant.
  • I then say $\pu{1 kg}$ is the approximate mass of $\pu{1 L}$ of water.
  • Does it mean I say the kilogram is defined as the mass of $\pu{1 L}$ of water? Of course it is not.
  • $\begingroup$ The subject and predicate should be reversed! One gram of matter contains approximately Avogadro's number of nucleons [protons + neutrons] [but not Avogadro's number of electrons unless the substance is protium or possibly the natural mix of H2] $\endgroup$
    – jimchmst
    Nov 25, 2022 at 23:29
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    $\begingroup$ @jimchmst Electrons were not talked about. :-) $\endgroup$
    – Poutnik
    Nov 26, 2022 at 0:14
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    $\begingroup$ @jimchmst Not really in this context. $\endgroup$
    – Poutnik
    Nov 29, 2022 at 22:58
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    $\begingroup$ @jimchmst If I say 1 kg is the approximate mass of 1 dm3 of water, it does not meant 1 kg is defined as the mass of 1 dm3 of water. $\endgroup$
    – Poutnik
    Nov 29, 2022 at 23:13
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    $\begingroup$ @jimchmst It is there just behind the definition. If he used a broader quote... If it fooled 1 person, than it is still great. Many correct statements fool multiple persons misinterpreting that. Especially if quoted out of context. $\endgroup$
    – Poutnik
    Dec 1, 2022 at 5:49

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