The Bohr radius (symbol: `a_0`) is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an atom.  It is expressed in units of meters (or very small pieces of meters).  It is used in the Atomic System of Units to derive other related atomic units such as velocity and force.

Notes

This constant is specified with a standard uncertainty (standard deviation) of: 0.00000000017 E-10 m

`a_0` = `(4pi*epsilon_0*h^2)/(m_e*e^2)`

    `m_e` - electron mass
     e - elementary charge
     h - reduced Planck's constant
     1/(`4pi*epsilon_0`) - Coulomb Force Constant