In chemistry classes or even many different branches of chemistry textbooks, I am told that the atomic symbol

$$\Large ^A_Z\ce{X}$$

includes the following notations:

  • $A$ – "atomic mass" or "mass number";
  • $Z$ – "atomic number".

But, because I want to make myself to remember easily, I would prefer to call $A$ "Atomic mass number" instead of simply "mass number". I think many say that I am making a big deal and unnecessary things. But as a non-native learner, if I can confirm that using the term "atomic mass number", it would make me easier to remember the term.


2 Answers 2


The IUPAC preference for A in $\ce{^A_ZX}$ is mass number. The term would also apply to a molecular fragment, for instance in a mass spec. So depending on the context atomic mass number or molecular mass number might be used to be more specific.

The problem with you arbitrarily defining A in $\ce{^A_ZX}$ as atomic mass number is that various references might just use mass number. So depending on context you'll need to be able to decide which possibility is the correct one. Of course in spite of IUPAC definitions there are no doubt multiple variations in the past and current literature.


Maybe I'm missing something, but I don't see how atomic mass number is easier to remember than just atomic mass. If it helps with distinguishing A from Z, Z is also called the proton number.

Not exactly a proof, but wikipedia includes "atomic mass number" as a synonym for mass number, so it is probably valid terminology.

  • $\begingroup$ In nuclear physics one would call A the number of nucleons. Of course this ties to the mass pretty directly (ignoring binding energy). $\endgroup$
    – Jon Custer
    Nov 22, 2017 at 13:55

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