For "nitrogen family" and "carbon family" the trend goes that in a period, the electron affinity in case of group 15 is less than that of group 14. This is attributed to the half-filled p-subshell which tends not to gain an electron. But this true till antimony only. Bismuth has a greater electron affinity as compared to lead.

My question is, why so? What is there that brings about this particular exception?

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    $\begingroup$ I think you got it backwards - it's the other ones that are exception. BTW I still don't get why people ask about electron affinity so often. $\endgroup$ – Mithoron May 8 at 21:07
  • $\begingroup$ In a sense this is an exception to an "exceptional trend". Looking at whole of the periodic table trends, the case of Pnictogen's electron affinity is an exception. But within this exceptional trend too, we find this case that contradicts what we would expect based on the ongoing trend. But the question remains that why such a contradiction is seen one would expect that owing to the half-filled p-subshell, bismuth would have less electron affinity. But it doesn't, why so? $\endgroup$ – Gurjot Singh May 10 at 12:54
  • $\begingroup$ Because those electrons are very weakly shielded by by the d and f electrons. They are just too tightly bound that the half filled logic just doesn't account for much. $\endgroup$ – Nisarg Bhavsar May 10 at 14:42
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    $\begingroup$ Relativistic effects play a role here. Electronic configurations ending with np2 are relativistically stabilised for large values of n (like [Xe]4f14 5d10 6s2 6p2 for lead). This is often called the "inert pair effect". The comments to this answer are related. $\endgroup$ – Nicolau Saker Neto May 10 at 22:33

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