Question

Why does 4s have a lower energy level than 3d in the electronic configuration?

Answer 1

The lower orbitals have a large energy gap in between - so the gap between 1s and 2s is huge. However, as you get higher and higher, the energy gaps start to diminish.

The difference in energy between 4s and 3d is actually very small - so small that as you fill the 4s orbitals, its energy becomes higher than 3d (and that, in a way, goes back and forth). Their energy levels are so similar that they can be considered equivalent - that's the reason that, for atoms such as chromium and copper, the electronic configuration is 3d5 4s1, and 3d10 4s1 - the energy level of the 3d orbital at that point is lower than the 4s. The explanation behind this is that there is a stabilization via symmetry - which is probably taught to you as the fact that half-filled and fully-filled orbitals are stable - and this is reflected by the energy levels that I mentioned.

These can be described fairly accurately when wavefunctions and Schrodinger's equations are introduced - but for all intents and purposes for you, all you need to know is that the orbital energy levels are not static.

--- Addendum ---

One way to think of this is that the s orbital penetrates nearer into the nucleus than the d orbital, so even though n = 4, it is nearer to the nucleus than the 3d orbital, thus filling up first.

Answer 2

http://answers.yahoo.com/question/index?qid=20110628155410AAcc7Qh
This explains it a little:
http://www.chemguide.co.uk/atoms/properties/atomorbs.html

And this section of the wikipedia article would help a little as well:
http://en.wikipedia.org/wiki/Electron_configuration#Shortcomings_of_the_Aufbau_principle

Answer 3

Thank you so much, that was really helpful!