= Isotope shift
An <isotope shift> is a difference between corresponding atomic transition <frequencies> in different <isotopes>. One contribution comes from nuclear <mass>, through recoil and electron correlations; another comes from the changing <nuclear charge distribution>. For a specified transition, the leading finite-size contribution is often written $\delta\nu_{\rm field}=F\,\delta\langle r^2\rangle$, with an electronic factor $F$. Extracting changes of <nuclear charge radius> therefore requires separating mass and finite-size contributions rather than identifying the entire shift with radius.
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