# Aufbau principle in filling Ti

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?

• The 3d/4s thing is much more complicated than textbooks would have you and I believe. The total energy of the system is not simply a sum of individual orbital energies (orbitals are quantum mechanical objects, and the maths doesn't work out that way). So, the fact that the aufbau principle works is a bit of a coincidence (and the "breakdown" of aufbau in some contexts should not come as a surprise). See e.g. chemistry.stackexchange.com/a/8426/16683 – orthocresol Jan 6 '19 at 17:01
• But, afaik, your lecturer's interpretation is correct. There are some references in the disclaimer here: chemistry.stackexchange.com/a/33310/16683 (I suggest not bothering with the rest of the answer.) – orthocresol Jan 6 '19 at 17:02