In 1913, Niels Bohr put forth his model of the structure of an atom. In this structure, he argued that electrons orbit the central nucleus, which is made up of protons and neutrons. The model went on to explain that electrons orbit at a specific distance from the nucleus and that the nucleus depends on the electrons’ energy.

The Bohr model attempted to explain that electrons follow a specific circular orbit. Electrostatic forces determine this orbit, therefore the electrons’ location can be pinpointed.

For example, hydrogen only has a single electron, which orbits the nucleus. Its smallest possible orbit with the lowest amount of energy is called the Bohr radius. What this determines is the most likely position of that electron.

The above equation can be used to calculate the Bohr radius by using other physical constants:

  • E0 is the permittivity of free space
  • h is the reduced Planck’s constant
  • Me is the electron rest mass
  • e is the elementary charge
  • C is the speed of light in a vacuum
  • Alpha is the fine structure constant.

It should be noted that the Bohr atom model does not correctly describe an atom. However, the Bohr radius is used as a characteristic size of the electron cloud when used in quantum-mechanical descriptions. In other words, it is used as a unit in atomic physics.

While the Bohr model is no longer accepted, it is typically used as an introduction for students who later learn about more accurate models. The Bohr radius is important.