Class 9 Chemistry Atoms and Molecules Atomic mass

Atomic mass

Atomic mass refers to the mass of an atom. It depicts how many times an atom of an element is heavier than one-twelth (1/12th) the mass of one atom of carbon-12 of mass of one carbon atom. The relative atomic masses of all elements have been established with reverence to an atom of carbon-12. It is measured in unit called amu (atomic mass unit). 1/12 of the mass of one atom of carbon is termed as relative atomic mass. It doesnot possess a unit.

Mathematically,

mass of one atom of an element = atomic mass X (1/12th) of the mass of one atom of carbon. 

Or

atomic mass = mass of one atom of an element / (1/12th) of the mass of one atom of carbon.

 

 

Easy way to calculate atomic mass of oxygen atom.

The Proton number Z = 8
The Neutron number N = 8
The atomic mass of the oxygen atom is given by 15.9994 amu i.e. nearly 16.

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