- 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