• Atoms of elements are very small.
  • To understand about atomic mass, it is a need to know about Carbon-12.
  • Carbon-12 can be stated as no. of protons= 6 & no. of neutrons= 6 thus making it as 12.
  • Carbon-12 atoms has been made as atomic mass of exactly 12 atomic mass unit.
  • Earlier the unit was ‘amu’ (atomic mass unit)
  • Now, the unit is ‘u’.
  • Atomic mass unit= 1/12th mass of carbon-12 atom.
  • The atomic mass of all other element is relative mass of its atom as taken with respect to the mass of carbon-12 (for atomic mass & mass no. kindly refer to structure of atom topic mass & atomic no.)
  • Through experiments, the mass of a Carbon-12 is found to be 1.9926 x 10ˉ²³ gram and when divided by 12, we get absolute mass of the atomic mass unit and it is 1.6605 x 10ˉ²⁴ gram.
  • Atoms do not exist in free state, they are either in the form of molecules or in the ionic forms.
  • Atoms of an elements are characterized by symbols to make them unique.
  • Different scientists had proposed symbols to the atom of an element.

Let us see about John Dalton’s symbol of element


  • It was difficult to use.
  • It was inconvenient to use.


  • This was done to make it more convenient.
  • J.J. BERZELIUS proposed that the 1st letter of the element should be used as its symbol.
  • The symbol can be two letter or one letter word. Ex: hydrogen is represented by H, Boron is represented by B.
  • There are many elements that begin with the same letter, for example Copper, Chlorine, Calcium. So different ways are assigned for naming them.
  • Names and symbols of  discovered elements are approved by IUPAC.
  • IUPAC stands for International Union of Pure and Applied Chemistry.
  • There are many elements which got the symbol form the Latin name. Here are few examples quoted in the table below.


  • Molecule is made up of group of atoms.
  • The molecules have chemical bonds between them.
  • Molecules can be formed with the combination of same elements or also with the different elements.


  • It is defined as the number of Atoms that constitute one molecule.
  • For Example O2 is made up of two molecules of oxygen, thus this will be called as diatomic.
  • CO2 is made up of 1 molecule of carbon & 2 molecules of oxygen, thus CO2 is tri-atomic.
  • When the term Polyatomic is used, it means when three or more atoms are together held. It can be S8 ( Sulphur molecule), CH4 (Methane)


  • The number of times a substance is higher than that of 1/12th mass of carbon atom. The molecular mass of oxygen is 32, which means that oxygen is 32 times heavier than that of 1/12th of a carbon atom.
  • The unit for molecular mass is ‘u’.
  • To calculate the molecular mass of a substance we need to…
  1. Find the mass of each atom present.
  2. Multiply the mass by the no. of atoms in them.
  3. Add all the numbers together.

Smriti Kanchan

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