The Aufbau Principle - Transition Elements

For the transition elements in the fourth period, the 4d and 3d electron addition order requires an explanation that goes beyond the aufbau principle for lighter elements.

Here is a table of the electron configurations for elements 19-36, the fourth row of the periodic table. The transition elements scandium-zinc are shown in pink. For these elements, the electron energy order and filling order may at first seem unusual.

Period 4: Potassium - Krypton

Element No.	Name	Electron Configuration
19	Potassium	[Ar] 4s1
20	Calcium	[Ar] 4s2
21	Scandium	[Ar] 3d1 4s2
22	Titanium	[Ar] 3d2 4s2
23	Vanadium	[Ar] 3d3 4s2
24	Chromium	[Ar] 3d5 4s1
25	Manganese	[Ar] 3d5 4s2
26	Iron	[Ar] 3d6 4s2
27	Cobalt	[Ar] 3d7 4s2
28	Nickel	[Ar] 3d7 4s2
29	Copper	[Ar] 3d10 4s1
30	Zinc	[Ar] 3d10 4s2
31	Gallium	[Ar] 3d10 4s2 4p1
32	Germanium	[Ar] 3d10 4s2 4p2
33	Arsenic	[Ar] 3d10 4s2 4p3
34	Selenium	[Ar] 3d10 4s2 4p4
35	Bromine	[Ar] 3d10 4s2 4p5
36	Krypton	[Ar] 3d10 4s2 4p6

4s vs 3d Sublevels

At scandium, rather than a valence electron configuration of 4s² 3d¹, we get 3d¹ 4s².

This order is experimentally verified. For example, if scandium ionizes, it is the 4s electrons that are lost, not the 3d electron.

Electron Configurations of Scandium and its Ions

Number of Electrons	Species	Electron Configuration
21	Sc	[Ar] 3d1 4s2
20	Sc+	[Ar] 3d1 4s1
19	Sc2+	[Ar] 3d1 4s0
18	Sc3+	[Ar] 3d0 4s0

Why do 4s and 3d Switch?

The energy levels of an atom's electrons are influenced by a number of parameters, such as the nuclear charge and electron-electron repulsions.

When there are fewer than 20 protons in the nucleus, the 4s sublevel is lower than the 3d sublevel.

However, for 20 protons or more, 3d drops below 4s.

Scandium has three valence electrons. The aufbau principle tells us they should add to the lowest available energy sublevel. Therefore, the first electron goes into 3d.

However, it turns out to be more favorable for the second to enter 4s. This is because when there are 20 protons or more, the 3d orbitals are more compact than 4s. The higher repulsion energy of a second electron entering 3d makes it unfavorable compared with the second electron entering 4s. Likewise, the third electron goes into 4s because it is the lowest cost energy option.

The Chromium Anomaly

Chromium has 24 protons and electrons. From the above, we might expect its electron configuration to be [Ar] 3d⁴ 4s².

In fact, it is [Ar] 3d⁵ 4s¹.

To explain this, again, we must bear in mind that the energy levels of the 3d and 4s are very similar. In chromium's case, the second option has lower energy because it the higher number of parallel spins produces the lowest energy configuration. This is in accordance with the principles of Hund's rule.