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How the Atomic Model Changes at A-Level

Understand the differences between the Year 10 and Year 12 atomic models

As an A-Level or Advanced Highers student you will have been introduced to a new atomic model, a new way of understanding the structure of atoms. Well, not totally new but it probably seems it. It’s so different from what we have previously learnt.

I discussed the differences and the reason for those differences in this podcast episode – listen to it here.

Why do we need to change the atomic model?

The model of the atom you have learnt at GCSE or Nat5 is very basic. It will have helped you get a basic grasp of the atom and the first few rows of the periodic table. It worked for the beginner level chemistry you were studying, but now you are embarking on more advanced studies.

As you look down the periodic table to the 4th period and beyond you see rows with 18 elements or 32 elements.

That wouldn’t be possible using the basic model of the atom The model where you learnt that there are a maximum of 8 electrons from the second shell onwards, and that would mean a maximum of 8 elements per period.

Another thing you probably wondered about was the two-dimensional nature of the atomic structure. The “solar system” model of electrons orbiting the nucleus is flat and convenient to draw, and it is also inaccurate. Or rather, it is an over-simplified depiction of atoms, one that doesn’t allow for the three-dimensional molecules we encounter all around us.

So we need a more advanced model that copes with the shape of the periodic table and the shape of molecules.

How the atomic model changes

We now introduce a few concepts to the atomic model, concepts that had previously been omitted for simplicity.

We introduce the concept that shells are divided into orbitals, and that the orbitals within a shell have slightly different energy from each other.

Above: Comparison of basic and more advanced atomic models.

These orbitals come in a variety of shapes which give an atom volume and make three-dimensional molecules possible. We will learn more about these shapes in a future episode, but here is a sneak preview:

In summary, we learn and consider the true nature of electrons – their behaviour and their position within atoms.

Why haven’t we considered the true nature of electrons prior to this? Because we omitted quantum mechanics from the simplified model we have been using. To consider electrons and atoms properly we have to introduce quantum mechanics to the model, and that does make it a bit more complicated. Don’t worry though, students just like you have thrived at this level. What’s more, we’re going to make understanding quantum mechanics as simple as possible starting in our next episode.

Got a Question for a Future Episode?

If you have a question or specific topic you would like to hear answered in an episode, just fill in the form and let me know. It’s questions like yours that will help shape the podcast, and I appreciate every single suggestion.