Bohr radius

https://dx.doi.org/10.1351/goldbook.B00693
Atomic fundamental physical constant used as atomic unit of length:
a 0 = 4 π ɛ 0 ℏ 2 m e e 2 = 5.291 772 49 24 × 10 −11 m
where e the elementary charge, ℏ the Planck constant divided by 2 π and m e the electron rest mass.
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