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I noticed this the other day, and rather than go hunt in some atomic physics book I thought I'd post it here where the answer will hopefully be more useful to the wider internets.

I was looking at this image, which came up on some social media feed or other, and I noticed that for the alkali and alkali earth metals, as well as for the whole of the p block on the right of the periodic table, the ionization energy tends to go decrease as you go down the table, but that for the transition metals it doesn't.

Image quoted from Compound Interest, licensed under CC-BY-NC-ND.

The first behaviour is sort of what you'd expect, with the outer shell further out and better shielded for bigger atoms, and therefore slightly easier to ionize. However, the transition metals seem buck this trend, and this is borne out by a closer look:

(Here the s block is in red, the p block in blue and the d block in black.)

I know that the transition metals are well known to have a bunch of such counter-intuitive behaviours (at least to intuitions honed on smallish atoms with few relativistic effects!), but these do tend to have accessible explanations in terms of relatively specific physical phenomena.

So, I would like to ask: is there a specific reason why the transition metals' ionization energies tend to increase with the period within each group?

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    $\begingroup$ Well, I'm a chemist, have never noticed this trend and am intrigued too. The d-block elements electron cloud of course builds up slightly differently than for s and p-block elements. For Period 3, the electron configuration is $[Ar]3d^{N-2}4s^2$ where $N$ is the group number, so e.g. $[Fe]=[Ar]3d^64s^2$. This 'inner filling' of the lower laying $d$ orbital is 'makes' them 'd-block'. So what you found must almost certainly be related to that specific electron structure. But how?? $\endgroup$
    – Gert
    Commented Feb 28, 2016 at 22:51
  • $\begingroup$ You don't however seem to have plotted the trend for all (10) d-block groups? $\endgroup$
    – Gert
    Commented Feb 28, 2016 at 22:55
  • $\begingroup$ @Gert Gah! good catch. Groups 11 and 12 do have a kink - they do go down from period 4 into 5 - but they still go up from period 5 to 6; I don't know how much that changes things but I think not much. Cd→Hg and Ag→Au are pretty special jumps on the Table, but from the naive intuition it would still be a fluffier atom and easier to ionize. (Obviously the naive intuition is very wrong, the question is how so.) $\endgroup$ Commented Feb 28, 2016 at 23:41
  • $\begingroup$ On your first comment, I guess it sort of depends (i) on whether the electron configuration for the first ion is $[\mathrm{Ar}]3d^{N-3}4s^2$ or whether you actually remove an $s$ electron (probably the former), and (ii) whether that structure remains for period 5 and 6. If it does, then you're still looking at how easy it is to ionize a $3d$ shell vs a $4d$ or a $5d$ shell one, with the attendant shielding of the inner shells, and I don't see how those $d$ shells would be all that different to the $p$ and $s$ shells on those same periods. $\endgroup$ Commented Feb 28, 2016 at 23:50
  • $\begingroup$ For sure they have an outer $s$ shell a bit further out, which would lessen the shielding but not its behaviour with bigger shells. (right?.) I was thinking this was likely relativistic effects, same as gold being yellow and mercury being liquid, but maybe there are some funky electronic effects to do with the shell inversion. $\endgroup$ Commented Feb 28, 2016 at 23:51

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It appears there isn't a single, satisfying explanation for this 'anomalous' trend but rather at least causes two that tentatively explain what Emilio Pisanty observed.

1. Lanthanide contraction:

At the elements 57 to 71, the lanthanides (or $1^{st}$) f-block, the $4f$ is being filled up with 14 electrons, rather than the $5d$ orbital. This causes the elements from $La$ to $Lu$ to actually contract.

Compare the ionisation energies of $Zr$ and $Hf$: $640\:\mathrm{kJ/mol}$ and $658\:\mathrm{kJ/mol}$ respectively. This type of difference is likely to make itself felt for $P5$ and $P6$ elements belonging to the same Groups, to the right of Group 4.

A similar contraction happens between elements 89 to 103, the actinides (or $2^{nd}$) f-block.

2. Deviations from the Aufbau Principle:

Consider this interactive Periodic Table and mouse over the elements for their electron configuration. In numerous cases the outer shell (valence electrons) configuration is not the expected $Ns^2$ but one electron has slipped into the lower $(N-1)d$ orbital. Compare e.g. $Nb$ to $Ta$ and $Mo$ to $W$. These anomalies are likely to affect the expected ground state energy and thus first ionisation energy.

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