Editing Atomic units (section)

== Physical constants ==
[[Dimensionless physical constant]]s retain their values in any system of units. Of note is the [[fine-structure constant]] {{tmath|1= \alpha = {e^2} / {(4 \pi \epsilon_0\,\hbar c)} \approx 1/137 }}, which appears in expressions as a consequence of the choice of units. For example, the numeric value of the [[speed of light]], expressed in atomic units, is {{tmath|1= c = 1/\alpha\,\text{a.u.} \approx 137\,\text{a.u.} }}<ref name="KarplusPorter1970">{{citation |last1=Karplus |first1=Martin |last2=Porter |first2=Richard Needham |year=1970 |title=Atoms and Molecules: An Introduction for Students of Physical Chemistry |location=Netherlands |publisher=W. A. Benjamin }}</ref>{{rp|597}}

{|class="wikitable"
|+ Some physical constants expressed in atomic units
|-
!scope="col"| Name
!scope="col"| Symbol/Definition
!scope="col"| Value in atomic units
|-
| [[speed of light]] || <math>c</math> || <math>(1/\alpha) \,a_0 E_\text{h}/\hbar \approx 137 \,a_0 E_\text{h}/\hbar</math>
|-
| [[classical electron radius]] || <math>r_\text{e}=\frac{1}{4\pi\epsilon_0}\frac{e^2}{m_\text{e} c^2}</math> || <math>\alpha^2 \,a_0 \approx 0.0000532 \,a_0</math>
|-
| [[reduced Compton wavelength]]<br/> of the electron || {{math|''ƛ''{{sub|e}}}} <math>= \frac{\hbar}{m_\text{e} c}</math> || <math>\alpha \,a_0 \approx 0.007297 \,a_0</math>
|-
| [[proton mass]] || <math>m_\text{p}</math> || <math>\approx 1836 \,m_\text{e}</math>
|-
|}