Bohr radius

<math>a_0 = {{4\pi\epsilon_0\hbar^2}\over{m_e e^2}}</math>

where:

<math>\epsilon_0</math> is the permittivity of vacuum

<math>\hbar</math> is the Planck's constant

<math>m_e</math> is the electron rest mass

<math>e</math> is the elementary charge

It has a value of 5.29189379×10^-11 m, or about half an angstrom.