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The mass of Carbon-12 atom is 12.0000000 a.m.u. For relative isotopic mass, e.g. Mg-24 atom is 24 compare with the c-12 as standard. However, why is it unitless? Shouldn't it be 24 a.m.u.?

Jessica
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  • Notice the term relative, meaning ratio of 2 quantities of the same unit. Note that the mass of the atom in units of 1/12 of 12C mass is not the same as it's relative mass, compared to this unity mass. – Poutnik Oct 25 '20 at 09:42
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    What is the unit of relative volume ? – Poutnik Oct 25 '20 at 10:13
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    Related: Units of mass on the atomic scale –  Oct 25 '20 at 10:37
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    Also note that the mass of a carbon-12 atom was not 12.0000000 amu. It was about 1.0003 amu (using the old definition of the atomic mass unit in physics) or 1.00006 amu (using the old definition of the atomic mass unit in chemistry). –  Oct 25 '20 at 10:41
  • I understand your query. So a atomic mass is equal to molar mass that is mass in grams per mole of the substance. it has the units grams per mole. This is because definition of mole says that it is the number of atoms of carbon in a sample of 12g of carbon-12. – Stack3002 Oct 25 '20 at 13:59
  • @Stack3002 Equal numerically only and only for a particular atomic mass unit. Atomic mass is the mass of an atom, in whatever units ( kg, amu,Da), (Atomic) molar mass is the mass of a mole of atoms. – Poutnik Oct 26 '20 at 06:22
  • My comment addresses only his query. He is referring to relative atomic mass when talking about mass in a.m.u. – Stack3002 Oct 26 '20 at 12:28

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Relative atomic mass is a dimensionless quantity. It is relative mass of a specie with respect to the mass of carbon-12. So the units of masses get cancelled and hence we have a 'unitless' quantity. Just like refractive index doesn't have a unit. Physical quantity that are a result of ratio don't have units as they get cancelled.

Desai
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