Using Slater's rule, I found that both lead and tin's $Z_\mathrm{eff}$ for 6p and 5p electrons equal. However, the ionisation energy is different. Is comparing ionisation enthalpy on the basis of $Z_\mathrm{eff}$ not correct?

Also, I found Clementi-Raimondi Effective Nuclear Charge which probably explains the trend, but why is this different to Slater's rule?

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    $\begingroup$ Perhaps Slater's rule is more of a suggestion. $\endgroup$
    – Jon Custer
    Commented Aug 19, 2019 at 18:27
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    $\begingroup$ This is an example of a relativistic effect, so Clementi-Raimondi effective nuclear charges (non-relativistic in nature) likely also cannot describe the behaviour adequately. You may find this question tangentially useful. $\endgroup$ Commented Aug 20, 2019 at 13:56


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