• A molecule is the smallest particle of a substance (element or compound) which has the properties of that substance and can exist in the Free State.
  • It is an electrically neutral group of two (or more) atoms chemically bonded together.
  • The force which holds the atoms together in a molecule is called bond.


Molecules of Elements:

  • The molecule of an element consists of two (or more) similar atoms chemically combined together.


For example:

  • A molecule of Nitrogen element contains 2 Nitrogen atoms combined together, and it is written as N2.
  • Nitrogen gas consists of N2 molecules and not of atoms of Nitrogen.
  • Hydrogen gas exists as H2 molecules, oxygen gas as O2 molecules and chlorine gas as Cl2 molecules.
  • The number of atoms present in one molecule of an element is called its atomicity.


Atomicity of some elements (redraw)

  • The atomicity of some of the common elements is:
  1. Noble gases (He, Ne, Ar and Kr) the atomicity is 1.
  2. The atomicity of hydrogen, nitrogen, oxygen, chlorine, bromine and iodine is 2 each.
  3. The atomicity of ozone is 3.
  4. The atomicity of phosphorus is 4.
  5. The atomicity of sulphur is 8.


Molecules of Compounds:

  • The molecule of a compound contains two (or more) different types of atoms chemically combined together in a simple whole number ratio and it is the smallest entity which has all the properties of a compound.


For example:

  • Hydrogen chloride or Hydrochloric acid contains atoms of Hydrogen and chlorine.
  • Water is a compound. A molecule of water (H2O) is made up of Hydrogen and Oxygen atoms.


Molecules of some compounds (redraw)