Currently, a mole is the amount of substance containing as many elementary entities as there are atoms in exactly 0.012 kilogram (or 12 grams) of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state. From May 2019, it is expected that one mole will contain exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol-1 and is called the Avogadro number.
The mole is used to describe a practical quantity of material and is the link between the microscopic and macroscopic worlds, used to scale phenomena from the atomic up to 'relevant' sizes. As a result of the definition, the mole contains a defined number of entities, usually atoms or molecules. This number is the Avogadro constant (NA).
The current value for NA is:
6.022 141 79(30)x1023 mol-1
This number is a dauntingly large figure.
- This number of sand grains would cover the United Kingdom to a depth of about a metre.
- There are about this number of human cells on Earth
- It would take you twenty thousand million million years to count this number of coins (counting about one coin per second)
The mole is not realised as a unique physical artefact like the prototype kilogram. However, it can be realised by certain reproducible experimental methods known as 'primary methods'. One example is by weighing a sample of material of known composition.