Bohr radius

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.

Source: