This is my understanding so far:

Average atomic mass is the mass of an element considering isotopes and is measured in atomic mass units. However relative atomic mass is the average mass of an atom ( Considering isotopes ) relative to carbon 12. As a result, relative atomic mass possesses no units. The same is applied to atomic mass which is the mass of an atom ( Isotope ) measured in atomic mass units. Relative isotopic mass is the mass of an isotope relative to carbon 12 and also possesses no units. Is this understanding completly correct?

Furthermore, I was wondering about Atomic mass units. If this measures the actual mass of an atom for example then how is it that relative mass gives the same result just without the unit. Is atomic mass units somehow related to carbon 12 as well?

  • $\begingroup$ The atomic mass unit is no longer related to C-12 because Avogadro's number is defined as a base quantity since the 2019 update of the SI units (and not anymore as the number of C-12 atoms in 12 gr of pure C-12). $\endgroup$
    – Paul
    Jan 19 at 7:14
  • 1
    $\begingroup$ en.wikipedia.org/wiki/… $\endgroup$
    – Buck Thorn
    Jan 19 at 14:39

1 Answer 1


It is a general topic of absolute and relative quantities.

Relative quantities are ratios of two quantities of the same unit, being unitless and independent on units.

Absolute quantities have numerical values related to the scale unit, depend on the chosen unit, which is the unit part of the quantity value.

The absolute and relative quantities can have the same numerical value, if the relative quantity reference is at the same time used as the scale unit of the absolute quantity.

Hypothetically, we could always use just relative quantities for everything, if it was always clear and accepted, what is the implicit reference. But we could not then use dimensional analysis for formal checks of equations.

If I take $\pu{1 kg}$ mass etalon as the relative mass reference, my absolute mass is e.g. $\pu{75 kg}$ and my relative mass is 75.

The same for atomic masses, if $\pu{1 Da}$ (Dalton) as the atomic mass unit is at the same time used as the reference for the relative atomic mass.


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