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According to Wikipedia, tennessine has an electron affinity (EA) of $\pu{+166 kJ mol-1}$ and nihonium $\pu{+67 kJ mol-1}$.

Normally this would make sense since tennessine is in group 17 and nihonium in group 13. However, the key feature of the 7th period is that the group 14 element is a pseudo-noble-gas and the group 18 element a psuedo-carbon-group-element; flerovium has a negative EA, like a noble gas, while oganesson tetrafluoride has Td symmetry, like the carbon group tetrafluorides. Therefore, nihonium, being one electron short of becoming "flerovium", should behave more like a halogen (and therefore have a more positive EA) than does the "true halogen" tennessine.

My question now follows - is this description on Wikipedia true, and, if true, why is it so?

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    $\begingroup$ It's not that cut and dried. Thanks to multiple relativistic effects on electron subshell energies, copernicium, flerovium, and oganesson all have configurations that may be potentially be called "closed-shell". No one of these elements is undisputably the "noble" one in Period 7. Indeed, the standard periodic predictions which are based on nonrelativistic quantum mechanics are breaking down at this point in the Periodic Table. $\endgroup$ Jul 29 at 10:03
  • $\begingroup$ @OscarLanzi Is this only an issue for Period 7? I.e., is radon unequivocally a nobel gas in the same way as those above it? $\endgroup$
    – theorist
    Jul 29 at 23:58
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    $\begingroup$ In fact it is not only an issue in Period 7. We see the mercury configuration developing a noble character in Period 6, where gold and even platinum can form anions with that configuration. Radon is also known in the +6 oxidation state corresponding to ... a mercury core. Period 7 is expected to be much more muddled, though, with its stronger relativistic effects $\endgroup$ Jul 30 at 0:07

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