I know , Average Atomic Mass of an element is - the sum of the Masses of its isotopes,each multiplied by its Relative Abundance percentage (in decimal notation).

But what confusing me is, what is the concept of Average Atomic Mass. In other words,why we want to take average atomic mass of every element on the periodic table,why not Atomic mass ? Why we need it and when to use it?

  • $\begingroup$ It depends on the branch of chemistry you are operating in, but for organic chemists calculating the molecular weight of molecules and molar quantities it is essential. $\endgroup$ – Waylander Mar 30 '20 at 9:01
  • $\begingroup$ Why should we need average atomic mass ? It is about the same question as : Why should we go to the Moon ? Why should we try and prove that the biggest prime number does not exist ? Why should we determine the number of beetles in the world ? Why should we study how Eratosthenes measured the radius of the Earth ? Why read Shakespeare ? $\endgroup$ – Maurice Mar 30 '20 at 9:49
  • $\begingroup$ If you want to separate the two peaks in mass spectrometry having the mass 28, you must know the precise atomic masses of C, N and O isotopes, because $^{12}C^{16}O$ seems to have the same mass as $^{14}N_2$. $\endgroup$ – Maurice Mar 30 '20 at 12:25
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    $\begingroup$ Suppose you have a bottle of NaBr and you need to use an amount equal to 1 mole. Sodium is all 23Na isotope, but bromine is 79Br and 81Br, with isotopic abundances 50.69% and 49.31%, respectively. So the NaBr in the bottle has both bromine isotopes and you cannot use the molar mass of NaBr with 79Br or the molar mass of NaBr with 81Br. You must use the average molar mass of NaBr if you want an accurate weighing of the NaBr. Only if you have isotopically pure chemical could you use the relevant non-average atomic isotopic weight. $\endgroup$ – Ed V Mar 30 '20 at 12:29

The average atomic mass is useful because its numerical value is equal to the molar mass of the element. This in turn is useful to know how much of a solid to take when you wish to react it with some known quantity of another reagent, because weighing is typically the easiest way to quantify a substance.


If you know precisely which isotope of the element you sre dealing with then go right ahead and use the molecular mass but generally we dont know which isotope is available to us and moreover there are mamy impurities a single gram might have a mixture of kany isotopes. So for reducing the error to the maximum limit possible statistically we multiply the mass of that particular isotope with its relative percentage abundance, you can imagine this as if a particular molecular mass is more available in nature than its more probable that is the isotope we have or that isotope is more in abundance in our mixture and hence the value of the average molecular mass we are using as simplification should tend more to that isotope


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