Electron affinity of atoms in the same row usually increases with the atomic number until the shell or half-shell gets full, and then it drops down. Some transition metals are exceptions from the rule, but the strangest exception to me is Titanium: while scandium ($3d^1$) has an affinity of 18 kJ/mol, titanium ($3d^2$) has an affinity of 8 kJ/mol.

It does not seem to make sense: the effective charge of titanium nucleus should be larger compared to scandium (Slater's rule), and exchange energy also favors titanium.

Does anyone have a good explanation?

  • $\begingroup$ No scandium has lower electro affinity because electro affinity increased with in a period and decrease with in a group $\endgroup$ Feb 24, 2021 at 10:40
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    $\begingroup$ @DagmawitAmanuel I'm afraid the experimental data I can find contradicts your assertion and supports the data presented by the OP. For instance en.wikipedia.org/wiki/Electron_affinity_(data_page) gives similar numbers to those quoted above, with references back to papers. Yes the original question would be MUCH better if the source for the numbers was given, but a bit of googling supports them. $\endgroup$
    – Ian Bush
    Feb 24, 2021 at 12:07
  • $\begingroup$ Note that it's similar, although more complicated with Lanthanum ([Xe]5d-1 6s-2) and Hafnium ([Xe]4f-14 5d-2 6s-2). The electron affinity of the former is higher. At the same time, the same exception does not apply to Yttrium ([Kr]4d-1 5s-2) and Zirconium ([Kr]4d-2 5s-2), but the interesting thing there is that Palladium ([Kr]4d-10) is close, and it has a different exception: a filled d sub-shell in the ground state of the atom, at the cost of a "harvested" s sub-shell. $\endgroup$
    – stanm
    Sep 17, 2022 at 12:26