Electronic Configuration

[futuredrummerdoctor]

The 4s orbital is lower in energy than the 3d orbital, and therefore you write 4s before 3d in the electronic configuration. Why, then, is the electronic configuration for Hafnium (Hf) the following?

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p64 d10 5s2 5p6 4f14 5d2 6s2 <-- Can someone please explain to me why 6s2 is written last? Shouldn't it be written as 6s2 5d2?

I think I'm missing something obvious- thanks!

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[5words]

Aubfau principle, electrons are supposed to occupy the lowest energy level (which is determined by the shell number) first. But i wouldn't worry about this, i doubt that the MCAT will even test for this.