I just learned that atoms of the same element can have different mass because of they are isotopes. If there are two isotopes in which each reacts to form the same thing. Would the mass differ in each of the two reactions? Is there a term for compounds just like the term isotope is for the atoms?

  • $\begingroup$ see:kinetic isotope effect $\endgroup$
    – A.K.
    Oct 22, 2020 at 10:18
  • $\begingroup$ The "quick questions" without explicitly expressed solving effort are not very welcome. That may lead to hesitation to answer or even to closing a question. If a question is asked on Chemistry SE site, the site policy expects authors to elaborate the question, e.g. by searching in textbook+online resources, writing what has been found, understood or tried and what is the stumble stone for the answer to be found. SE network aims for a collection of high quality answers AND questions. $\endgroup$
    – Poutnik
    Oct 22, 2020 at 10:31
  • $\begingroup$ It means that "I have just learnt that..." is wrong approach, implying you have not even thought about it much yet nor spent time to search for answers. $\endgroup$
    – Poutnik
    Oct 22, 2020 at 10:34
  • 3
    $\begingroup$ @Poutnik Sorry to say, but I actually did some google searches before. I did searched: "isotopes of molecule", "can the same molecule differs in mass", and many others. Please stop making assumptions. $\endgroup$
    – HilbertDJ
    Oct 22, 2020 at 10:38
  • 2
    $\begingroup$ These molecule are called isotopologues. $\endgroup$
    – Paul
    Oct 22, 2020 at 10:48

1 Answer 1


If 2 molecules have different isotopes of some element their masses will differ. In this respect there are 2 different terms:

  • Molecular mass a.k.a. molecular weight (MW) - is an average mass of a molecule. If we take many molecules (a mole of molecules) of the same substance: some of them will have isotope A, others - isotope B. Their average is going to be MW*. It's used in calculations e.g. when we want to know the mass (in grams) of N moles of some substances. Or vice versa - given the mass (in grams) we'd like to calculate how many moles of there are.
  • Exact molecular weight - the mass of the molecule with isotope A or isotope B. These are the masses that will differ.

*Note that in atmosphere the isotopes may come in different ratios. E.g. $Cl$ comes as 2 isotopes ($^{35}Cl$ and $^{37}Cl$) - their ratio is 3:1. So MW is not the average of $35$ and $37$, it needs to take the abundance into consideration: $\frac{0.75\times35 + 0.25\times37}{2}$


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