Well, the new term has started, new text books have been bought, your classes are full of new and eager students and you are starting your teaching of the new course. NOS in particular. So how is it going? Have you come up with any good examples of your own or are you focussing in the explanations in the guide?

One idea that came to my mind is that of Occam’s razor and teaching students about atomic structure and s,p,d,f orbitals (we now need to teach this to our SL students – don’t forget that!)


Occam’s razor is covered in section 2.7, it states:

The principle of Occam’s razor is used as a guide to developing a theory. The theory should be as simple as possible while maximizing explanatory power.” [1]

Back to the electron arrangement, you know the story… I was explaining to my students about how the model of electron arrangement that they were used to was a simplification (I don’t like to use the word wrong!) – We were using sodium as an example and the fact that it was not [2,8,1] but 1s2 2s2 2p6 3s1.

I try to get my students to come up with their own explanation (pattern) of how the number of subshells are related to each other. I explain that the first energy level has 1 sub shell, the second energy level 4 sub shells, the third energy level 9 subshells and so on. I then ask the students to come up with a mathematical relationship. This is when the fun starts!

All sorts of imaginative and complicated models are derived – for example (and you may have heard this one):

“List the numbers of subshells: 1, 4, 9, 16

Look at the difference between them 3, 5, 7

This is the pattern …..”

Ok, I reply, can you put this into a mathematical equation – this is then when they struggle (as well as me).

The key to ‘getting’ the pattern is to look for simplicity and efficiency – and this is Occam’s razor. The correct answer would be to let ‘n’ = the principle energy level.

The number of subshells is merely n2.

And it works.

Every time.

So there we have it – Occam’s razor in your own classroom – maybe not as the IB expected it to be used but there all the same and in a form that the students can relate to.

Or, in the words of the great Sherlock Holmes:

“When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” [2]

In fact Albert Einstein used it in order to determine parts of his theory on special relativity – so it can’t be such a bad this after all!

I would love to read some of your examples of how you have used the NOS in your teaching – please feel free to post them below.


[1] IBO Chemistry Guide, first exams 2016.

[2] http://www.bestofsherlock.com/top-10-sherlock-quotes.htm#impossible