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 carbon-12.**

**The Avogadro’s Constant**

12 grams of carbon-12 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 carbon-12. 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 carbon-12 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 carbon-12. 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 |