Atoms are very small and numerous to be counted one at a time. Instead the quantity of atoms is measured by mass. The unit of measurement for atoms and molecules is the mole. The mole is also the S.I unit for chemical quantity. The symbol for the mole is mol.
A mole of a substance contains the same number of particles as the number of atoms in 12 grams of carbon12.
The Avogadro’s Constant
12 grams of carbon12 contains 602 000 000 000 000 000 000 000 carbon atoms. This huge number is called Avogadro’s constant. It can also be written as 6.02×10^{23}. (The 10^{23} tells us to move the decimal point 23 places to the right, to get the full number.)
There are 6.02×10^{23} particles in one mole of a substance. The particles could be atoms, molecules, ions or electrons. So you can say that 1 mole of particles contains 6.02×10^{23} particles.
This is an atom of carbon12. It is chosen as the standard atom. Its Ar is taken as 12. Then other atoms are compared with it.
This is magnesium atom. It is twice as heavy as carbon12 atom. So its Ar is 24.
So you can say that 24 grams of magnesium contains the same number of atoms as 12 grams of carbon12. 24 grams of magnesium is also called a mole of magnesium atoms.
Examples
Sodium is made of single sodium atom. Its symbol is Na and Ar is 23.
The 23g of sodium contains 6.02×10^{23}sodium atoms or 1 mole of sodium atoms.
Iodine is made of iodine molecules. Formula of iodine is I_{2 }and Mr is 254.
The 254g of iodine contains 6.02×10^{23} iodine molecules of 1 mole of iodine molecule.
Conclusion
So you can see that one mole of a substance can obtained by weighing out the Ar or Mr of that substance in grams.
How can you find the mass of a mole?
You can easily find out the mass of one mole of any substance by following these steps
 First of all write down the symbol or formula of the substance
 Find the Ar or Mr of the substance
 Express the mass in grams
Substance

Symbol

Ar

Mr

Mass of one mole

Sodium

Na

Na = 23

Exists as single atoms

23 grams

Chlorine

Cl_{2}

Cl = 35.5

2 x 35.5 = 71

71 grams

Propanol

C_{3}H_{7}OH

C = 12
H = 1
O = 16

3 x 12 = 36
8 x 1 = 8
1 x 16 = 16

60 grams
