Electron configuration anomalies

by Ms. Monir

This may be you:  Ahhhhh!!!!!   I feel like I just figured out how electron configuration works, and now you tell me things like Fe3+ doesn’t follow the same rules?!  What do you mean when you say this is the electron configuration?!  That’s not Aufbau!  Somebody help me!


In class, you learned that the energy levels of electron configuration fill up according to their filling order which can be determined by the diagonal rule.


This is exactly how the periodic table is arranged, because the valence electrons go up the same way on the periodic table.  For example, the valence electron for Calcium which has 20 electrons is 4s2, but Scandium which has one more electron has a valence electron of 3d1.


D-Block Elements

Some elements, however, do not follow this filling order exactly.  When a transition metal loses electrons to form a cation, the first two electrons to be removed are the s ones, then the d.  Why?  It’s a little complicated, but the following may help make logic of these anomalies:

1. Completely filled sublevels ( like the d sublevel) are more stable than partially filled sublevels.
2. A sublevel which is exactly half filled is more stable than a partially filled sublevel which is not half full.
3. Electrons are “lazy” and will do whatever places them in the lowest energy state = which is the most stable state

Remember, even though in the filling order we put d electrons after p, a 3d electron is still in the same shell as 3p, and 4s is further away.  If we can half fill 3d, it will fill BEFORE 4s.

These anomalies generally involve the promotion of an s electron to the next d shell in order to achieve a full or half-full d shell.


Grouped by periods and shown in correct order of orbital filling

Chromium [Ar] 3d5 4s1
Copper [Ar] 3d10 4s1
Niobium [Kr] 4d4 5s1
Molybdenum [Kr] 4d5 5s1
Ruthenium [Kr] 4d7 5s1
Rhodium [Kr] 4d8 5s1
Palladium [Kr] 4d10 5s0
Silver [Kr] 4d10 5s1
Lanthanum [Xe] 5d1 6s2
Cerium [Xe] 4f1 5d1 6s2
Gadolinium [Xe] 4f7 5d1 6s2
Platinum [Xe] 4f14 5d9 6s1
Gold [Xe] 4f14 5d10 6s1
Actinium [Rn] 6d1 7s2
Thorium [Rn] 6d2 7s2
Protactinium [Rn] 5f2 6d1 7s2
Uranium [Rn] 5f3 6d1 7s2
Neptunium [Rn] 5f4 6d1 7s2
Curium [Rn] 5f7 6d1 7s2

There are also various exceptions in the f-block elements. The reasoning is somewhat similar.


Also see this article for more info:  The Trouble with the Aufbau principle