Masses of atoms are very small ranging from 10^-24g to 10^-22g and individual atoms are too small to be seen even by the most powerful microscope and impossible to weigh individually.When chemists need to know how many atoms are involved, they must count by weight large numbers. This is similar to the normal method of counting coins in a bank. A large number of coins are weighted together.

Definitions

The moles is that amount of substance which contains the same number of particles or entities( atoms, molecules, ions, electrons, protons, neutrons) as they are atoms in exactly 12g of carbon-12 –isotope

The Avogadro Constant is the number of particles or entities as there are atoms in exactly 12g of carbon-12 – isotopes. This is the number of particles in one mole of any substance.

When we weigh out the relative atomic mass of an element in grams we have a mole of atoms. Similarly the relative molecular mass in grams gives a mole of entities.

Although the mole is defined in terms of an amount of substances, it is more useful to treat it as meaning a huge number with a special name. For example:

A mole of carbon atoms, C, has a mass of 12g

A mole of copper atoms, Cu, has a mass of 63.5g

A mole of hydrogen atoms,H, has a mass of 1g