Relative atomic mass, Atomic mass unit and Average atomic mass

Relative atomic mass, Atomic mass unit and Average atomic mass

Q. Write a note on the following

  1. Relative atomic mass
  2. Atomic mass unit
  3.  Average atomic mass



The mass of one atom of an element is compared with one-twelfth [1/12] the mass of one atom of carbon-12.


The mass of one atom of an element when compared with the lightest isotope of carbon is known as relative atomic mass. It is expressed in grams or amu . Thus one gram atom of an element expressed in grams is known as relative atomic mass

An atom is an extremely small particle therefore it is impossible to find the exact mass of an element using a sensitive balance. The standard chosen is carbon whose mass is exactly 12g or 12amu.

Mass of one Carbon-12 atom=12amu

1 amu=mass of one C-12 atom/12

Now finding the relative atomic mass of hydrogen


=1.008 amu


One atom of sodium = 23 amu

 One atom of chlorine = 35.5 amu

One atom of calcium = 40 amu


The unit is used to express atomic and molecular masses equivalent to 1/12 to the mass of one carbon atom.


1 mol of C12=12g = 6.023×1023

1/12 × mass of one atom of C12 exactly as 12 = 1 amu

1 amu = 1/12 g/mol   /   6.023×1023 mol

1 amu = 1.66 × 10-24g = 1.66 ×10-27kg

The mass of 1 proton is equal to an amu.


The weighted average of atomic masses of naturally occurring isotopes of an element is known as average atomic mass.


In the majority of cases, the atomic mass of any element is rarely found to be in whole numbers. It is because most of the elements contain different isotopes and the relative atomic mass takes into account the natural abundance of each isotope

Average atomic mass = Atomic mass of 1st isotope × its % abundance/100 + Atomic mass of 2nd isotope × Its % abundance /100



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