Proton charge radius
= Proton charge radius
{title2=$r_p$}
The root mean square radius of the <proton> charge distribution. Finite nuclear size changes <hydrogen atom> <energy levels> and enters precision predictions for the <Lamb shift>.<Theory of the Lamb shift in hydrogen and light hydrogen-like ions by Vladimir Yerokhin and others>[§V A, Eq. (45) and definition of the root mean square nuclear charge radius]{ref}