# Highest naturally occuring binding energy of electrons

I was wondering which element has the highest binding energy of an electron. Is it simply the 1s electron of the heaviest stable element? If so, can somebody tell me where I can find a table of binding energies?

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The question is at least a little bit indeterminate because of "naturally occurring". For any given nucleus, the more highly ionized it becomes the greater the binding energy of the remaining electrons, culminating with getting the last one off of the hydrogen-like core, which runs roughly $13.6\frac{Z^2}{n^2}\text{ eV}$. Unfortunately for significant $Z$ those kinds of energies may be big enough to disrupt heavy nuclei which may come apart before they lose that last electron.
For a list of theoretical and experimentally observed lines, ask NIST. The highest value I find tabulated is a $\mathrm{Fm}$ K-edge at around 141 keV.