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Atomic Mass


  • Atom is made up of 3 sub-atomic particles i.e. electron, proton & neutron. Every object that exists in 3-D has mass. Atom to have mass and the  mass of an atom is very small.
  • The mass of an atom is calculated by using sophisticated techniques like mass spectrometry.
  • But in the 19th century, scientists could easily determine the mass of one atom relative to another atom.
  • Initially, the scientist took 1/16 of the mass of a naturally occurring oxygen.
  • The main reason for taking oxygen was that oxygen could react with a large number of elements, and by using Oxygen-16 the masses of many elements came as whole numbers.
  • In 1961 carbon-12 was taken as the standard and had been agreed upon.

Relative Atomic Mass

The average mass of an atom compared to the 1/12th mass of a Carbon-12 atom.

Atomic Mass Unit

  • Its is a mass unit.
  • 1 amu is defined as a mass which is exactly equal to one-twelfth the mass of one carbon-12.
  • 1amu = 1.66056*10^-24 g
  • amu is usually replaced by 'u' which is known as unified mass.

Elements and Their Atomic Masses

Hydrogen     = 1u
Helium         = 4u
Carbon         = 12u
Nitrogen      =  14u
Oxygen        = 16u
Sodium        = 23u
Magnesium  = 24u
Sulphur        = 32u
Chlorine      = 35.5u
Potassium    = 39u
Calcium       = 40u