Wednesday, September 26, 2018 0

I’ve just finished teaching this topic to my students. I always find it an interesting topic to teach as the ideas used are so different to those you are probably taught prior to IB.

Just when you were happy with the idea that atoms have electrons in shells you suddenly get told that this is not the case and that you need to know and use another model of the atom. It’s a great example of a paradigm shift (you will cover these in TOK). A paradigm shift, in a nutshell, is where one idea gets replaced with another and I find them really interesting.

Back to the idea of energy levels, I’m talking s, p, d orbitals, the shapes of these orbitals and the filling of these orbitals.

I tell to my students to first of all think of the ‘old’ idea of electron arrangement (i.e. Sodium is 2, 8, 1) to be a bit like looking at an atom through a low resolution microscope. Then a high power microscope comes along and it allows you to see the shells in greater detail. The second shell is in fact made of four mini energy levels called orbitals (an ‘s’ orbital and three ‘p’ orbitals). The third shell is made of nine energy levels (one ‘s’, three ‘ps’ and five ‘ds’).

At this point the class imagines these orbitals to be circular, just like the representation using the old model. Then I introduce the idea of the orbitals having different shapes and that really blows them out of the water!

I also use this idea to explain why the 4s fills before the 3d. The fourth energy level is higher in energy than the third but the 4s part of the 4th shell overlaps the outermost part of the 3rd shell (the 3d orbital) so it is actually closer to the nucleus than the 3d but overall is higher in energy. I think they understand this!

This is also a great topic to introduce trends in ionization energies as evidence for the arrangement of electrons in atoms – we all teach ionization energies but I think we overlook the point that they provided evidence for the arrangement of electrons in atoms. I always make sure that I emphasize this point to my classes so that the students can see the relevance of ionization energies – I don’t want them to see it as an abstract, stand-alone concept.

How do you teach the idea of energy levels? I’d love to hear from others so please feel free to share your ideas below.

Image credit

Geek3, via Wikimedia Commons (CC BY-SA 4.0)