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?