Counting by the dozen
Taking things in groups to make them easier to count is nothing new to you. Chemistry does the same thing — the group is just very much bigger.
| Unit | What it counts | How many |
|---|---|---|
| Dozen | Books, eggs | 12 |
| Ream | Sheets of paper | 500 |
| Mole | Atoms, molecules, ions | 6.022 × 1023 |
It is an official unit
Measuring the amount of a substance is needed in all sorts of work. The unit used for measuring amount of substance in the International System of Units is the mole.
It does not suit everything
Because the mole stands for such a large amount, it does not suit measuring most of the substances met in daily life. In practice the mole is used for measuring the amount of things that occur in enormous numbers — atoms, molecules and ions.
It is the idea of the dozen
A dozen books is 12 books, whatever sort of book. A mole is 6.022 × 1023 particles, whatever the particle. So "a mole of sodium" and "a mole of glucose" are the same count, not the same mass.