The Convention

What I should say to start with is that we’ve followed a convention for electron configurations so that, if there are electrons present in orbitals, lower principal quantum numbers are always shown preceding higher principal quantum numbers in the configuration. This means that it doesn’t matter whether in a real atom the 5s has lower or higher energy than the 4d orbitals. We always write 4d before 5s.

In the case of iodine this convention actually does yield the correct electron configuration – although it may at first seem to disagree with the orbital energy levels shown in textbooks, which show the filling order for orbitals as:

1s
2s 2p
3s 3p
4s 3d 4p
5s 4d 5p
6s 4f 5d 6p
7s 5f 6d 7p

Why [Kr] 4d10 5s2 5p5 is Iodine’s Electron Configuration

If we apply the Aufbau principal to the orbitals above, we would predict that the 5s orbital, because it has lower energy than the 4d orbitals, will fill with electrons before the 4d orbitals do – as you’ve said.

In fact, when two electrons are present in the 5s orbital the energy of the 4d orbitals falls below the energy of 5s. Therefore, the correct configuration for iodine is [Kr] 4d10 5s2 5p5

The reason we don’t always get the result we’d expect from applying the Aufbau Principal to orbitals is that when real electrons begin to interact with one another, some shifts in orbital energy levels can take place.

Thanks for the interesting comment Jessica – it may be worth considering whether we continue to show configurations using the principal quantum number convention or whether we show the actual configuration, when it’s known.