Aufbau principle in filling Ti

Question

A few weeks ago, I was given this question to complete:

The ground state electronic configurations of Ti is $\ce{[Ar] 4s^2 3d^2}$
State whether it follows the Aufbau principle or not. If not, explain why and state what the configuration would be if the principle was followed.

Now, I know the Aufbau principle tells us that when filling orbitals with electrons, the lowest energy orbital is filled first.

Looking at this diagram and the periodic table, I thought the filling was right - $\ce{4s}$ is before $\ce{3d}$ so I thought it was reasonable.

However, according to the Lecturer this is apparently wrong.

He says:

The $\ce{3d}$ level is at a lower energy than the $\ce{4s}$ and should be filled first according to the principle. It takes the above configuration to minimise the d-d interelectronic repulsion energies. The alternative configuration of [Ar]3d4 would have a higher energy.

I'm slightly confused here.

How am I wrong?

Answer 1

The energy level diagram which you give and is to be seen in the standard textbooks is assumed to apply to all elements. This is not the case, nor would we expect it to be so. After calcium the 3d drops below the 4s so fills first. Coulombic repulsions tend to push electrons up into the larger 4s orbital where repulsion is less. This means the 4s electrons are the outer electrons which are lost first in chemical reactions and define the atomic radius of the atom.