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bohr radius
The bohr radius is 5.29177210544 × 10⁻¹¹ metres, the most probable distance between the proton and the electron in a hydrogen atom's ground state. It is the length unit of atomic physics: express the Schrödinger equation in bohr radii and hartrees and the constants disappear from it.
Watch out: The value is measured, not defined, and this is the CODATA 2022 figure. Quantum chemistry output often reports lengths in bohr with no unit label at all, and reading them as angstroms understates every bond length by about 47%.
| 1 a0 | 5.2917721e-11 m |
Where the unit came from
It comes from Niels Bohr's 1913 model, where the electron orbited at fixed radii. The model is superseded, but the radius survived because the quantum-mechanical hydrogen ground state peaks at exactly the same distance.
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