Molecule

Molecule

A molecule is the smallest particle of an element or compound that can exist independently. E.g. molecule of water (compound), sodium chloride (compound), molecule of oxygen(element), etc.

There are many elements which have only one atom in one molecule, e.g. copper, silver, lead, mercury etc.

Although most elements can exist independently but atom of some elements cannot exist independently and their molecules have two atoms. These elements are – oxygen, hydrogen, nitrogen, chlorine, iodine, bromine and fluorine.

Some common molecules

1. Baking soda –  NaHCO₃
2. Hydrous copper sulphate – CuSO₄.5H₂O
3. Gypsum – CaSO₄.2H₂O
4. Plaster of Paris – CaSO₄.1/2H₂O

How to calculate molecular number

H₂SO₄ = 2×1+32+16×4 = 98
HNO₃ = 1+14+16×3 = 63
HCl = 1+35.5 = 36.5

Avogadro’s number (or Avogadro’s constant)
It’s defined as number of atoms in 12 grams of the isotope carbon-12.
6.022 × (10)²³ = 12 gram of Carbon
It is also defined as number of atoms of an element in 1 gram of that element, i.e. 1 gram of hydrogen contains 6.022 x 10²³ atoms of hydrogen