The unit of length in the atomic system of units. It is equal to the Bohr radius, which is approximately the radius of maximum electron density in the hydrogen atom, about 5.291 772 085 9 × 10⁻¹¹ meter, with a 1 standard deviation uncertainty of ± 0.000 003 6.¹ Symbol, a₀.²

The bohr = 4πεħ²/mee², where me is the mass of the electron, e is the charge of the proton, and ε₀ is the electric constant, what used to be called the permittivity of the vacuum.

1. 2006 CODATA value.

2. E. R. Cohen, T. Cvitas, J. G. Frey, B. Holmström, K. Kuchitsu, R. Marquardt, I. Mills, F. Pavese, M. Quack, J. Stohner, H. L. Strauss, M. Takami, and A.J. Thor.
Quantities, Units and Symbols in Physical Chemistry.
IUPAC Green Book, 3rd Edition, 2nd Printing.
Cambridge, UK: IUPAC & RSC Publishing, 2008.
Pages 135, 183.

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