mole
/ mohl /
We have everyday words for counting in bundles: a pair is 2, a dozen is 12, a gross is 144. The mole is just a counting word for an enormous bundle — the chemist's 'dozen' for atoms and molecules. Because atoms are so unimaginably tiny, you need a colossal number of them before you have enough to weigh and pour, and the mole is the size of that bundle.
A mole is the SI unit for amount of substance. One mole contains exactly 6.02214076 × 10²³ elementary entities (atoms, molecules, ions, or whatever you specify) — this fixed count is the Avogadro number. The beauty of the mole is the bridge it builds: one mole of any substance has a mass in grams equal to its molar mass (the formula weight read off the periodic table), so weighing out grams becomes a way of counting particles.
The mole matters because chemistry happens particle-by-particle — reactions combine atoms in whole-number ratios — yet we can only weigh bulk amounts. The mole lets us translate between the two: grams in the flask and atoms in the reaction. A common confusion to clear up: a mole counts particles, not mass, so a mole of lead and a mole of hydrogen contain the same number of atoms but have very different weights, just as a dozen elephants and a dozen mice are both twelve.
Water has a molar mass of about 18 g/mol, so 18 grams of water is one mole — roughly 6.022 × 10²³ molecules. That is about a tablespoon of water holding more molecules than there are stars in the observable universe.
A tablespoon of water is about one mole of molecules.
Always say what entities you mean: a 'mole of oxygen' is ambiguous — it could mean a mole of O atoms or a mole of O₂ molecules, which differ in mass by a factor of two.