Atomic fundamental physical constant used as @A00504@ of length: $a_{0}=\frac{4\ \pi \ \varepsilon_{0}\ \mathrm{\hslash }^{2}}{m_{\text{e}}\ e^{2}}=5.291 \ 772\ 49\ (24)\times 10^{-11}\ \text{m}$ where $$e$$ is the @E02032@, $$\mathrm{\hslash}$$ is the @P04685@ divided by $$2\ \pi$$ and $$m_{\text{e}}$$ is the @E02008@.