A polished silicon sphere reflects the light, a glint slides across it. Inside, a lattice of atoms is visible, gently breathing. The letters mol show through the sphere. On the right, a pinch of graphite drops onto the balance pan; the balance shows 12.000 g and flashes. Above, a counter of particles blinks.
Quantity data sheet
SI base unitrev. 2026-10Mole mol
SI base unit · quantity: amount of substance — a number of particles counted in dozens of 6·10²³
The sphere whose atoms were counted
In 2011 metrologists from several countries completed a silicon sphere weighing a kilogram. It is round to within a few tens of nanometres. By measuring its diameter and the spacing of its crystal lattice, they counted the atoms in it. This gave the most precise value of the Avogadro constant.
Atoms are too small to count one by one. A chemist takes them in portions. One portion — a mole — is exactly 6.022 140 76·10²³ particles. The number is chosen so that a mole of carbon-12 atoms weighs 12 grams. That is why the mass of a mole of any substance is easy to read from the periodic table.
A mole of water is 18 grams, a little less than a tablespoon. A mole of table salt is 58 grams. A mole of any gas at room temperature takes up about 24.8 litres, roughly two buckets.
The symbol of the mole is mol, three lowercase letters, with no full stop and no endings: “5 mol”. Amount of substance is written with a lowercase n. The Avogadro constant is N_A, molar mass a capital M.
A mole is a number of particles, not a mass. A mole of hydrogen weighs 2 g, a mole of gold 197 g, yet they hold the same number of particles. Do not confuse it with molarity M — moles per litre of solution — or with the “gram-molecule”, an old name for the same mass.
01 · Definition
number · mass · gas volumeA mole is a count, not a weight. Just as a dozen is twelve items, a mole is 6.022 140 76·10²³ items. The items can be atoms, molecules, ions or electrons, and they are always named: a mole of oxygen atoms and a mole of oxygen molecules are different amounts.
To weigh out a mole you need the molar mass — the mass of one mole. It equals the relative atomic or molecular mass in grams. Carbon is 12.011 g/mol, water 18.015 g/mol. Divide the mass of the sample by the molar mass and you get the number of moles.
Since 2019 the mole has been defined by a number: the Avogadro constant is fixed exactly, with no uncertainty. The fact that a mole of carbon-12 weighs 12 grams is no longer a definition but a measured fact — very precise, but no longer exact by agreement.
How many particles are on the balance pan
On the left is an analytical balance with a glass draft shield. Choose a substance and set the mass: the balance shows grams, and the counter shows moles and the number of particles. In the magnifier on the right is a handful of particles, enlarged so that single atoms are visible. “Weigh out a mole” pours exactly one mole. For a gas, a syringe replaces the balance: the volume of a mole grows with temperature. Blue shows mass and the balance, teal the amount of substance, crimson the particles. Below are three views. “Mass and moles” is the straight line m = n·M for different substances. “Particles” shows how many items are in different amounts, on a logarithmic scale. “Gas” is the volume of a mole against temperature.
A mole of sand grains would cover a continent
If every particle of a mole were a grain of sand a millimetre across, a mole of sand would take up 6·10¹⁴ cubic metres. A layer like that could cover all of North America to a depth of a few tens of metres. And 18 grams of water — one gulp — holds as many molecules.
02 · Conversion
mass · particles · solutionsFrom grams to moles and to items
Enter a mass, moles or a number of particles. The first tab converts them for the substance chosen in the scene. The second lists molar masses. The third gives solution concentrations in moles per litre.
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The mole is handy because it hides an enormous number. A single cell holds about an attomole of a protein; the oceans hold about 7·10²² moles of water. The same count covers everything from a few molecules to a whole planet, and the chemist works in the middle — with millimoles and moles.
04 · Measuring instruments
balance · burette · silicon sphere · mass spectrometerMoles are not counted — they are weighed and titrated
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05 · Writing rules
mol · which particlesSay a mole of what
The mole symbol is not inflected or abbreviated. Next to the amount, name the particles: a mole of atoms, molecules or ions. Molar mass is written in grams per mole, concentration in moles per litre.
The symbol has no plural. Molar mass without “/mol” turns into a mass. M already means moles per litre, so “/L” is redundant. The prefix is lowercase, and the symbol has no capitals. The Avogadro constant has a unit, mol⁻¹. “A mole of oxygen” is ambiguous: atoms or O₂ molecules — a factor of two.
06 · Neighbouring units
chemistryThe same silicon sphere serves the kilogram too.
mol/LMoles in a litre of solution.
katA mole per second — enzyme activity.
07 · Historical section
archive · from Avogadro’s hypothesis to an exact numberHow the invisible was counted
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The mole became exact, and 12 grams did not
Before 2019 the mole was defined as the number of atoms in 12 grams of carbon-12. The mass was exact, and the Avogadro number was measured, with an uncertainty. Now it is the other way round: the number is fixed, and the molar mass of carbon-12 is measured. It equals 12 grams per mole with a relative uncertainty of about 4·10⁻¹⁰.
For a chemist nothing changed: molar masses in the tables stayed the same. But for metrology it matters. The mole no longer depends on the kilogram or on a particular isotope. To measure out a mole you do not have to weigh — you can count.
On the right are two pans. Under the old definition the mole was tied to a 12-gram weight. Under the new one the weight is the same, but next to it stands a counter, and the counter is what counts.
Catalogue of quantities
Substance and chemistry · 12 unit sheets
Index · all quantities on one pageThe lab at the entrance to the indexA drop of acid changes the colour of the juice, salt colours the flame, water freezes at a tap. Below them are all Simetrium sheets, by discipline and alphabetically.Open the index →
