Bohr radius

Bohr's theory of the hydrogen atom (1913) predicts the existence of a smallest orbit for the electron circulating the hydrogen nucleus. Today the radius of this orbit is called the Bohr radius. It is usually indicated by a0.

In the old quantum theory of Bohr and Arnold Sommerfeld, as well as in the new quantum theory of Werner Heisenberg and Erwin Schrödinger the radius is given by
 * $$ a_0 = \frac{4\pi \epsilon_0 \hbar^2}{\mu e^2} \approx 0.529\,177\,208\,59\cdot 10^{-10}\; \mathrm{m} $$

where &epsilon;0 is the vacuum permittivity (electric constant), $$\hbar$$ is Planck's reduced constant, &mu; is the reduced mass of the hydrogen atom (is equal to the electron mass when the proton mass may supposed to be infinite; for the numerical value given this assumption is made) and e is the charge of the electron.

In quantum mechanics,  a0 appears  as the maximum in the radial distribution associated with the electronic wave function &Psi;1s(r) of lowest energy of the hydrogen atom, the  1s atomic orbital. That is, a0 is the position of the maximum in  the radial distribution 4&pi;r2 |&Psi;1s(r)&thinsp;|2 and in that sense a0 is a measure for the "size" of the hydrogen atom.

External link
NIST value for bohr radius