The Bohr radius is given by $a_0 = \frac{4 \pi \varepsilon_0 \hbar^2}{m_e e^2}$, where $\varepsilon_0$ is the permittivity of free space, $\hbar$ is the reduced Planck's constant, $m_e$ is the mass of the electron, and $e$ is the charge of the electron.
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