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?