Borh

In the Bohr model of the structure of an atom, put forward by Niels Bohr in 1913, electrons orbit a central nucleus. The model says that the electrons orbit only at certain distances from the nucleus, depending on their energy. In the simplest atom, hydrogen, a single electron orbits the nucleus and its smallest possible orbit, with lowest energy, has an orbital radius almost equal to the Bohr radius. (It is not exactly the Bohr radius due to the reduced mass effect. They differ by about 0.1%.)

Although the Bohr model is no longer in use, the Bohr radius remains very useful in atomic physics calculations, due in part to its simple relationship with other fundamental constants. (This is why it is defined using the true electron mass rather than the reduced mass, as mentioned above.) For example, it is the unit of length in atomic units.

According to the modern, quantum-mechanical understanding of the hydrogen atom, the average distance − its expectation value − between electron and proton is ≈1.5a0,[3][note 2] somewhat different than the value in the Bohr model (≈a0), but certainly of the same order of magnitude.
The Bohr radius of the electron is one of a trio of related units of length, the other two being the Compton wavelength of the electron \lambda_{\mathrm{e}} \ and the classical electron radius r_{\mathrm{e}} \ . The Bohr radius is built from the electron mass m_{\mathrm{e}}, Planck's constant \hbar \ and the electron charge e \ . The Compton wavelength is built from m_{\mathrm{e}} \ , \hbar \ and the speed of light c \ . The classical electron radius is built from m_{\mathrm{e}} \ , c \ and e \ . Any one of these three lengths can be written in terms of any other using the fine structure constant \alpha \ :where:

\lambda_{\mathrm{p}} \ is the Compton wavelength of the proton. \lambda_{\mathrm{e}} \ is the Compton wavelength of the electron. \alpha \ is the fine structure constant.
In the above equation, the effect of the