# Difference between atomic mass, mass number, atomic weight, and relative atomic mass [duplicate]

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I think this question is common, but i want a little help. I'm little confused. What I concluded.

Relative atomic mass/Atomic weight (both are exactly the same) - It is the mass of atom calculated as taking the sum of all the masses of the subatomic particles and dividing it with mass of (the same technique calculated) 1/12th of C-12 atom.

Atomic mass- It is the mass of the atom calculated as taking the mean according to their abundances in nature, of atomic weight of the different isotopes present in that sample.

Mass Number- It is the mass calculated as the sum of the nucleons , as there is only a slight difference in the mass of the proton and neutron, and the mass of electron is negligible as compared with the mass of the proton and neutron

But this is conflicting with definitions on wikipedia. Also the relative atomic of carbon should be exactly 12, but its 12.011

## marked as duplicate by Jan, Todd Minehardt, ringo, orthocresol♦, Jon CusterSep 21 '16 at 16:05

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• Well, the atomic mass of carbon deviates from 12 because not all carbon in nature is C-12. As for the rest, your understanding is mostly right; what exactly contradicts it? – Ivan Neretin Jul 30 '16 at 7:27
• wikipedia says, atomic mass and the relative mass are different but, it also says both are calculated as relative to 1/12th of the C-12 atom. Then where is the difference. – Adesh Tamrakar Jul 30 '16 at 8:24
• – Loong Jul 30 '16 at 14:04
• I can't even decide which is the correct duplicate. grumbles – orthocresol Sep 21 '16 at 16:01

## 1 Answer

Please correct me if I am wrong but in my understanding, the following are the definitions for the aforementioned terms:

Mass number: The total number of nucleons present in the nucleus of an atom(Dimensionless).

Atomic mass: The combined mass of all the nucleons present in the atom of any given element(Unit: amu/u).

Relative isotopic mass: Ratio of atomic mass to one amu(Dimensionless)

Average atomic mass: Weighted average of all the isotopes for the given element(amu/u).

Relative atomic mass/atomic weight: Ratio of atomic weight to one amu(Dimensionless).

Here is the Wikipedia page, explaining that "atomic weight" is simply the older term used for "relative atomic mass":

Relative atomic mass