Why is ionization energy of indium less than gallium?

In group 13 we observe an irregular trend in ionization energy: B > Tl > Ga > Al > In.

Gallium has a filled 3d subshell, but indium has a filled 4d and 3d subshell. Thus it should have more poor shielding along with more ionization energy, but that is not the case why?

• – Nilay Ghosh May 6 '20 at 5:59