Unit of length is the Bohr radius: The first is in atomic units, second in SI-units. This quantity can be remembered by recalling the virial theorem, i.e. that in absolute value, half of the potential energy is equal to the kinetic energy. This gives us and if we accept this relation, we have the above value of .
The so called fine structure constant expresses in general the weakness of electromagnetic interaction.
Some Constants and Quantities
m s Bohr velocity
m Bohr radius
eV s Planck's constant
eV '27-0.2emK Boltzmann constant
R = 8.3143 J mol '27-0.2emK