Why is it that the relative atomic mass of an atom is numerically equal to gram atomic mass?

For example,Relative Atomic Mass of Oxygen= 16 u So,Gram atomic mass of Oxygen=16 g.


1 Answer 1


The relative atomic mass of an atom is its mass relative to an atom of carbon-12 (which is 12 by definition). A mole of carbon 12 atoms weighs 12g (which is where the definition comes from).

"Gram atomic mass" is just an alternative way for saying the mass of a mole of some atom. So the ratio of the atomic masses will always be the same regardless of how many of the atoms you are counting. Hence, the two ideas are numerically the same.

  • $\begingroup$ I think that there is a flaw in your answer. Your Justification was only TRUE for C12 atom as you never talked or related other atoms.How would you justify it for all the other atoms? $\endgroup$
    – user42209
    Commented Mar 10, 2017 at 19:08
  • $\begingroup$ @user42209 Because other atoms atomic masses are defined relative to carbon 12 which is the standard. This makes what I said true (at least to a good approximation). $\endgroup$
    – matt_black
    Commented Mar 11, 2017 at 0:48

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