Molecules

• 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)