What is Relative atomic mass? Amu, Scale, Calculations


“The ratio of the average mass of one atom of an element to one-twelfth of the mass of an atom of carbon-12.”
  • The elements of the periodic table sorted by atomic mass Atomic Mass Name chemical element Symbol
  • 1.0079 Hydrogen H
  • 4.0026 Helium He
  • 6.941 Lithium Li
  • 9.0122 Beryllium Be

Atomic Mass Unit (AMU)

“The unit used to express the relative atomic mass is called the atomic mass unit (AMU) and it is 1/12th of the mass of one carbon atom.

Example: On the carbon-12 scale, the relative atomic mass of 126c is 12.0000 amu and the relative atomic mass of H is 1.008 amu.

1 amu is = 1.661×10−27 kg.


  • The relative atomic mass (amu) of an element is calculated from the mass numbers of its isotopes. The abundance of these isotopes.
  • You can always find the relative mass of an element by adding the number of protons to the number of neutrons for the specific isotope of the element you’re considering. For example, a carbon-12 atom has 6 protons and 6 neutrons, and so has a relative atomic mass of 12.

Relative atomic mass scale

The relative atomic mass scale is now based on an isotope of carbon. namely, carbon-12, nuclide symbol, which is given the arbitrary value of 12.0000 amu by international agreement. The unit ‘amu’ is now being replaced by a lower case u. Where u is the symbol for the unified atomic mass unit. Similarly, we use amu or atomic mass unit to measure the relative atomic mass of an atom. For example, the relative atomic mass of Hydrogen is 1 amu. The relative atomic mass of Oxygen is 16 amu.

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