2. A pair What do these have in common?
What do these have in common? Each group is a group of two, also called a pair.

3. A dozen What do these have in common? What do these have in common:
12 eggs
12 hats
Each group is a group of twelve, also called a dozen.
A dozen

4. 6,022 x 1023 particles A mole of particles
What do these have in common?
6,022 x 1023 particles
What do these have in common:
12 g carbon
253,8 g iodine
18 g water
Enough tennis balls to have the same volume as the moon
They all consist of the same number of particles: 6,022 x 1023 particles. This is also called a mole of particles. Mole is abbreviated as mol.
A mole of particles
Mole is abbreviated as mol

5. Why do we need the mole?
The mole is a large group: a group of 6,022 x 1023 particles.
Let us look at two reasons why we need the concept of the mole. These are:
- It is easier to work with groups of small things, rather than individual units.
- The mole concept allows us to calculate how many particles we have of a substance, by knowing the mass we have of the substance.

6. Why do we buy milk in litres, rather than in drops?

7. Why do we buy eggs in dozens rather than in ones?

8. Why do we refer to how many moles we have of a chemical, rather than how many particles we have of the chemical? It is easier to work with groups of small things, rather than individual units.

9. Imagine you have a big bag of 10c coins
Imagine you have a big bag of 10c coins. How can you find out how many coins you have? You could count them, but that would take a long time. It would be quicker if you knew the mass of one 10c coin, and measured the mass of all the coins. From this you could calculate how many coins you had. Try this: Let’s pretend that each 10c coin has a mass of 2g. All your coins together have a mass of 200g. How many coins do you have? You have 100 coins.

10. Notice that it is possible to use the mass of what we have to find out the number of particles we have, as long as we know the unit mass (in other words the mass of one).
Imagine you had a big bag of rice grains. How can you find out how many rice grains you have? Again, it is possible to count them, but it will be difficult. It will be much easier if you know the mass of one rice grain, and the mass of all your rice grains, and from this calculate how many rice grains you have. It will be much easier to calculate number from mass, than to count.

11. Now imagine you are told to count how many carbon atoms there are here, or how many iodine molecules there are here, or how many water molecules there are here. Now it is not only easier to calculate number from mass – it is the only way to find out the number of particles, because it is impossible to count! It is impossible to count numbers of atoms and molecules because there are so many of them even in a small mass of substance, and because they are so tiny they cannot be seen.
The mole concept allows us to calculate how many particles we have of a substance, by knowing the mass we have of the substance.

12. What does the word mole mean?
6,022 x 1023 is a very large number.
 000
What is a mole?
What does the word mole mean?
The mole is a large group: a group of 6,022 x 1023 particles. Here we have:
6,022 x 1023 atoms of Carbon (a mole of Carbon atoms)
6,022 x 1023 molecules of Iodine (a mole of Iodine molecules)
6,022 x 1023 molecules of water (a mole of water molecules)
6,022 x 1023 tennis balls (a mole of tennis balls)
6,022 x 1023 is a very large number. Here we have it written out in full:
 000
How would we say this large number in words? We could say: 602 thousand, 200 million million million.
602 thousand, 200 million million million

13. Avogadro’s number A mole consists of 6,022 x 1023 particles
A mole consists of 6,022 x 1023 particles. The number 6,022 x 1023 has been called by a special name, since it is a special number. It is called Avagadro’s number.
How big is Avogadro’s number? It is about 1x1014 times bigger than the world’s population: in other words, 6,022x1023 is about 100 million million times more than the number of people living in the world at the moment. That means that if you had all the people living in the world, 100 million million times, then you would have as many people as there are Carbon atoms here. That is a lot of Carbon atoms! From this you should see that atoms are very very small!

14. Avogadro’s number Mole = 6.022 x 1023
Pairs
Dozen
Avogadro’s number
Mole = x 1023
In the past, people chose the word ‘pair’ to refer to ‘two’, because things often come in pairs: for example shoes, and gloves. People chose the word ‘dozen’ to refer to ‘twelve’, because it was an easy to sell eggs in groups of twelve.
Why did Avogadro choose 6,022 x 1023 to be the special number which can also be called a mole?

15. To understand this, we need to remind ourselves about some information we can get from the periodic table. This is Carbon’s information from a periodic table. Carbon’s atomic number is 6. This tells us that Carbon is the 6th element in the periodic table, and that every Carbon atom has 6 protons in its nucleus. Carbon’s mass number is 12. This means that most Carbon atoms have 12 nucleons in their nuclei: 6 protons and 6 neutrons. Each nucleon has a mass of 1 atomic mass unit, abbreviated as amu, or u. So most Carbon atoms have a mass of 12 amu.

16. Avogadro’s number was chosen so that the mass number has another meaning too. When measured in grams, an element’s mass number is the mass of one mole of atoms of that element. 12 g of Carbon is 1 mole of Carbon atoms: 6,022 x 1023 Carbon atoms. 32g of Sulfur is 1 mole of Sulfur atoms. If we want 6,022 x 1023 Iron atoms, all we need to do is measure out 56g of Iron. When used in this way, the mass number for an element is called that element’s molar mass.
Notice that this allows us to calculate how many particles we have by measuring the mass of what we have.