Electron radius

The classical electron radius is a combination of fundamental physical quantities that define a length scale for problems involving electrons interacting with electromagnetic radiation. According to modern understanding, the electron is a point particle with a point charge and no spatial extent. Attempts to model the electron as a non-point particle are considered ill-conceived and counter-pedagogic. Nevertheless, it is useful to define a length that arises in electron interactions in atomic-scale problems. The classical electron radius is given as (in SI units)

where ${\displaystyle e}$ and ${\displaystyle m_{\text{e}}}$ are the electric charge and the mass of the electron, ${\displaystyle c}$ is the speed of light, and ${\displaystyle \varepsilon _{0}}$ is the permittivity of free space. This numerical value is several times larger than the radius of the proton.

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