Formula Classical Electron Radius Radius Mass Elementary charge Speed of light Vacuum Permittivity
$$r_{\text e} ~=~ \frac{e^2}{8\pi \, \varepsilon_0 \, m_{\text e} \, c^2} ~\approx~ 1.4 \,\cdot\, 10^{-15} \, \text{m}$$ $$r_{\text e} ~=~ \frac{e^2}{8\pi \, \varepsilon_0 \, m_{\text e} \, c^2} ~\approx~ 1.4 \,\cdot\, 10^{-15} \, \text{m}$$ $$m_{\text e} ~=~ \frac{e^2}{8\pi \, \varepsilon_0 \, r_{\text e} \, c^2}
$$ $$e ~=~ c \, \sqrt{ 8\pi \, \varepsilon_0 \, m_{\text e} \, r_{\text e} }$$ $$c ~=~ \sqrt{ \frac{ e^2 }{ 8\pi \, r_{\text e} \, \varepsilon_0 \, m_{\text e}} }$$ $$\varepsilon_0 ~=~ \frac{e^2}{8\pi \, r_{\text e} \, m_{\text e} \, c^2}$$
Radius
$$ r_{\text e} $$ Unit $$ \mathrm{m} $$ Radius of the electron derived in the classical way (i.e., without taking quantum mechanics into account) (although there are several different formulas for the classical electron radius). Note that it is just a theoretical model value and not an experimentally determined quantity!
Mass
$$ m_{\text e} $$ Unit $$ \mathrm{kg} $$ Rest mass of electron is a physical constant and has the value: \( m_{\text e} ~=~ 9.109 \cdot 10^{-31} \, \text{kg} \).
Elementary charge
$$ e $$ Unit $$ \mathrm{C} = \mathrm{As} $$ Elementary charge is the charge of the electron and has the value: \( e ~=~ 1.602 \cdot 10^{-19} \, \text{C} \).
Speed of light
$$ c $$ Unit $$ \frac{\mathrm m}{\mathrm s} $$ Speed of light is a physical constant and has the value in vacuum: \( c = 299 \, 792 \, 458 \, \frac{\text m}{\text s} \).
Vacuum Permittivity
$$ \varepsilon_0 $$ Unit $$ \frac{\mathrm{As}}{\mathrm{Vm}} $$ Vacuum permittivity is a physical constant with the value: \( \varepsilon_0 = 8.854 \cdot 10^{-12} \, \frac{ \text{As} }{ \text{Vm} } \).