Sometimes it is important to know exactly how many particles (e.g. atoms or molecules) are in a sample of a substance, or what quantity of a substance is needed for a chemical reaction to take place.
The amount of substance is so important in chemistry that it is given it's own name, which is the mole.
 Definition 1: Mole
The mole (abbreviation 'n') is the SI (Standard International) unit for 'amount of substance'.
Now that we know what a mole is, we can relate it to something that we know already. This is the relative atomic mass. For example, if we have a sample containing 1g of hydrogen then we have 1 mole of hydrogen, since the relative atomic mass of hydrogen is 1u.
We can build up to the idea of Avogadro's number. For example, if you have 12 eggs then you have a dozen eggs. After this number we get a gross of eggs, which is 144 eggs. Finally if we wanted one mole of eggs this would be 6,022× 10236,022×1023. That is a lot of eggs!
In one mole of any substance, there are 6,022× 10236,022×1023 particles. When we talk about the mole, we should always say what the particles are. The particles can be atoms, molecules, electrons, or almost anything else.
 Definition 2: Avogadro's number
The number of particles in a mole, equal to 6,022× 10236,022×1023.
If we were to write out Avogadro's number then it would look like:
602200000000000000000000602200000000000000000000. This is a very large number. If we had this number of cold drink cans, then we could cover the surface of the earth to a depth of over 300km300km! If you could count atoms at a rate of 10 million per second, then it would take you 2 billion years to count the atoms in one mole!
The original hypothesis that was proposed by Amadeo Avogadro was that 'equal volumes of gases, at the same temperature and pressure, contain the same number of molecules'. His ideas were not accepted by the scientific community and it was only four years after his death, that his original hypothesis was accepted and that it became known as 'Avogadro's Law'. In honour of his contribution to science, the number of particles in one mole was named Avogadro's number.

Complete the following table:
Table 1Element  Relative atomic mass (u)  Sample mass (g)  Number of moles in the sample 
Hydrogen  1.01  1.01  
Magnesium  24.31  24.31  
Carbon  12.01  24.02  
Chlorine  35.45  70.9  
Nitrogen   42.08  
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How many atoms are there in...
 1 mole of a substance
 2 moles of calcium
 5 moles of phosphorus
 24,31g24,31g of magnesium
 24,02g24,02g of carbon
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