The Bohr radius 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. The precise definition of the Bohr radius is: where: Or, in Gaussian units the Bohr radius is simply According to 2010 CODATA the Bohr radius has a value of 5.2917721092×10^−11 m. 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. 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. For example, it is the unit of length in atomic units. Wikipedia [ - ]

The Bohr radius is a physical constant, approximately equal to the most probable distance... [ + ]