Atomos em n mols (Avogadro)

Avogadro's constant: N = n · N_A

The 2019 revision of the SI pinned Avogadro's constant to an exact value, N_A = 6.02214076·10²³ entities per mole. The mole now comes straight from N_A instead of from carbon-12. The old wording said 12 g of pure ¹²C held exactly 1 mol of atoms, and the number works out the same either way. Going from moles to particles is just multiplication by N_A. Take 0.5 mol of water: 0.5 · 6.022·10²³ ≈ 3.011·10²³ molecules. The other direction works too, so 1.2046·10²⁴ atoms / N_A = 2 mol.

Applications

It shows up when you count atoms and molecules in stoichiometry problems, in statistical physics through the Boltzmann distribution and partition functions, in the signal intensity of nuclear magnetic resonance, in the dosimetry of radiopharmaceuticals, and in electrochemistry, where the Faraday constant is N_A · e.

FAQ

Why is N_A exact now? From 2019 on, the SI fixes the value of N_A and defines the mole from it. The kilogram likewise hangs off Planck's constant, which is how the base units stopped depending on physical artefacts.

Does N_A apply only to atoms? No. It counts whatever entity you name, whether that's atoms, molecules, ions, electrons, photons or formula units.

How big is 6·10²³? Picture spreading 1 mol of sand grains evenly across all of Brazil. The layer would pile up a few kilometres deep.