Mass of atom is called atomic mass. Since, atoms are very small consequently actual mass of an atom is very small. For example the actual mass of one atom of hydrogen is equal to 1.673 x 10^{-24} g. This is equal to 0.000000000000000000000001673 gram. To deal with such small nuber is very difficult. Thus for convenience relative atomic mass is used.

Carbon-12 is considered as unit to calculate atomic mass. Carbon-12 is an isotope of carbon. The relative mass of all atoms are found with respect to C-12.

One atomic mass = 1/12 of the mass of one atom of C-12.

This means atomic mass unit = 1/12th of carbon-12

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Thus atomic mass is the relative atomic mass of an atom with respect to 1/12 th of the mass of carbon-12 atom.
‘amu’ is the abbreviation of Atomic mass unit, but now it is denoted just by ‘u’.

The atomic mass of hydrogen atom = 1u.

This means one hydrogen atom is 1 times heavier than 1/12^{th} of the carbon atom.

The atomic mass of oxygen is 16u, this means one atom of oxygen is 16 times heavier than 1/12^{th}of carbon atom.

Absolute mass or Actual atomic mass:

It is found that, the actual atomic mass of a carbon-12 atom is equal to `1.9926 xx 10^(-23)`g.

`:. 1u =(1.9926xx10^(-23))/12 g`

`=>1\ u = 1.6605 xx 10^(-24)g`

Thus by multiplying the relative atomic mass with 1.6605 × 10^{-24}g we can get the absolute or actual mass of an atom.

Example (1) Find the absolute mass oxygen (O).

Solution:

The atomic mass of oxygen is 16u

We know that, `1u=1.6605xx10^(-24)`g

Therefore, Absolute mass of oxygen

`=1.6605 xx 10^(-24)xx16`g

`=26.568xx10^(-24)`g

`=2.6568 xx 10^(-25)`g

Example (2) Find the absolute mass of Sodium (Na).

Solution:

The atomic mass of Sodium = 23u

Since, `1u=1.6605xx10^(-24)`g

∴ Absolute mass of Sodium

`=1.6605xx10^(-24)xx23` g

`=38.191 xx 10^(-24)` g

`=3.8191 xx 10^(-25)` g

Example: (3) Calculate the absolute mass of hydrogen (H) atom.

Solution:

And the atomicu mass of hydrogen (H) = 1 u

∵ `1\ u= 1.6605 xx 10^(-24)` g

∴ Absolute mass of Hydrogen (H)

`=1.6605 xx 10^(-24) xx 1\ ` g

`=1.6605 xx 10^(-24)` g

Example:(4) Find the absolute mass of Nitrogen(N) atom