What is 1 amu? From where it is derived? Why it is compared with carbon-12? Why it was once compared with hydrogen-1? and now its not? Can anyone please explain me logically? And what would have happened if other elements were used? May be my question is wrong. If it is, can anyone point my mistake out?

  • 3
    $\begingroup$ What do you mean by "detailed logical solution"? And did even see en.wikipedia.org/wiki/Atomic_mass_unit ? $\endgroup$
    – Mithoron
    Oct 17, 2015 at 18:45
  • $\begingroup$ I think I read a bit but I am not getting $\endgroup$
    – Ahmad
    Oct 17, 2015 at 18:48
  • $\begingroup$ What is 1 amu? I think it was originated from hydrogen-1? I think, it is like hydrogen-1 has a value of 1 amu? So, why oxygen 16 was later used? $\endgroup$
    – Ahmad
    Oct 17, 2015 at 18:50
  • $\begingroup$ how 1 amu is derived? $\endgroup$
    – Ahmad
    Oct 17, 2015 at 18:52
  • 1
    $\begingroup$ Please tell us exactly what you didn't understand. Repeating what Wikipedia said is useful to none. $\endgroup$
    – M.A.R.
    Oct 17, 2015 at 19:39

1 Answer 1


Originally, atomic mass had been just relative and without any units. In 1803, John Dalton decided to use hydrogen-1 as a basis for atomic mass, but Wilhelm Ostwald disagreed later on, and said that relative atomic mass should be measured relative to 1/16 of an oxygen-16 atom. This was, however, before elemental isotopes were discovered.

When isotopes of oxygen were discovered in 1923, relative atomic masses had different representations based on the isotopes used. In physics, pure oxygen-16 was given an RAM of 16 amu, whilst chemists gave this value to naturally occurring, isotopically-weighted oxygen.

These different values led to errors in computations, and so in 1961, the reference standard was changed to carbon-12. Carbon-12 was used to stop any more divergence in values for RAM. The new (and current) unit became u, the "unified atomic mass unit".

Therefore, the atomic mass unit is 1/12 of carbon-12, and so the mass of carbon-12 is 12.

  • $\begingroup$ Thanks but why they did not consider a single isotope of hydrogen? $\endgroup$
    – Ahmad
    Oct 17, 2015 at 19:29
  • $\begingroup$ It's low mass leads to inaccuracy, and so a heavier element was used so that error decreases. $\endgroup$ Oct 17, 2015 at 19:40
  • $\begingroup$ why would it? or why they did not consider a single isotope of oxygen? What is the point of amu, since we can determine the mass in amu just by getting the nucleon number? $\endgroup$
    – Ahmad
    Oct 17, 2015 at 19:44
  • $\begingroup$ A low mass increases percentage error, making it less accurate. As I said in my answer, they did consider oxygen, but before isotopes were discovered. We use amu as a unit for atomic masses. The problem with just using nucleons number is the existence of isotopes. $\endgroup$ Oct 17, 2015 at 19:54
  • $\begingroup$ So. we can consider one specific isotope instead of a specific element? Like oxygen-16, what is to do with isotopes? $\endgroup$
    – Ahmad
    Oct 17, 2015 at 20:13

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