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Transcript: The Atomic Model for A Level Chemistry

Matthew Macariou:

Hello, welcome to this week’s episode of the Chemistry Made Simple podcast. I’m Matthew Macariou from Chemistry Made Simple.

This week, we’re talking about atomic structure. So in this episode, we’re going to be talking about why we need a more advanced atomic model than the one we’ve learned so far. What are the differences between what we’ve learned already and what we need to advance to now, and what concepts we need to introduce in order to make that leap.

Why the Model Needs to Change

Firstly, we need to consider why we need to have a more advanced model at all. The basic model has some limitations. It only works for the basic concepts that we’ve learnt so far. It doesn’t really explain the shape of the periodic table, for example, you’ll have learned that from the second shell onwards shells are able to accommodate up to 8 electrons.

But if you look at the periodic table you’ll see that from the fourth period, from the fourth row down the periodic table, you begin to see more than eight elements across the periodic table. In fact, you see 18 in the fourth period and you see even more further down the periodic table. And because you already know that each consecutive element has one more electron than the previous element, you’ll realize that that means there must be more than eight electrons that are available in those shells. So we need to advance our model in order to be able to understand that. If there’s 18 elements, of course, there must be room for 18 electrons in a shell to go across that period.

Another issue is that the model that we’ve dealt with is very flat. It’s very two-dimensional. I consider it as a solar system type model. So in our basic model so far, we’ve considered that around the nucleus of the atom, we have shells for electrons that orbit around that nucleus in a very two-dimensional way. It’s a very flat concept.

That’s nice and easy to draw on a two-dimensional piece of paper, but doesn’t actually reflect the three-dimensional nature of atoms.

Comparison of A Level vs GCSE Atomic Models

Let’s make a comparison between the basic atomic model that you will have learnt so far. What I’m calling the year 10 model here, and the year 12 model, the more advanced model that you are learning now. In the basic model, we understood that the nucleus of an atom contains the protons and the neutrons, and we’re not changing that concept. The nucleus does contain the protons and the neutrons. In our basic model, we considered that the first shell is able to accommodate up to two electrons. And the second and subsequent shells are able to accommodate up to eight electrons each.

And the first part of that is true. The first shell is able to accommodate up to two electrons. The second shell of an atom is able to accommodate up to eight electrons, but subsequent shells have space for more than eight electrons. And actually each subsequent shell has more capacity to accommodate electrons than the previous shell. There are no two shells or an atom that have the same capacity for electrons.

In our basic model, we gave no consideration to any division of shells. We just considered that all of the electrons in any shell had the same energy as each other and behaved in exactly the same way as each other as well. But we’re going to introduce in this model the concept that electron shells are made up of orbitals and actually the orbitals within a single electron shell can have slightly different energy and slightly different behavior to each other.

In summary, our basic model was fine for the basic concepts that we’ve considered in chemistry so far, and also for the first 20 elements on the periodic table. Certainly when we go beyond element 20 and we start looking at slightly more advanced concepts in chemistry, that we need to advance this model and we can start to see the effect of more electronic capacity per shell and the effect of having orbitals within shells. So what are these new concepts that we’re introducing?

New Concepts in the A Level Atomic Model

Well, we’ve already mentioned that shells are comprised of orbitals and that orbitals within a shell have electrons with slightly different energy levels and slightly different behavior. But it must follow that orbitals within a shell have to be different types to each other. And actually these have different shapes as well. And you’ll come to be able to recognize and even sketch some of the basic shapes of some of the orbitals. And all these things are because we’re now introducing the concept of quantum mechanics into the atomic model. In other words, we stop ignoring quantum mechanics. They’re always there, they’re always affecting the atom. We’ve just left them out of our model so far, but now we have to introduce them in order to start understanding some of the concepts of chemistry.

So what is quantum mechanics? Well, it’s a branch of physics. It deals with the behavior of extremely small particles. And in chemistry, we’re going to consider how quantum mechanics affects, describes the behavior of electrons in particular. Quantum mechanics can also be applied to many other small particles. It can be applied to atoms, it can be applied to photons and many other very small particles. These particles have very counterintuitive behavior and properties compared to the everyday world we see around us. We’re going to focus on quantum mechanics in our next episode so don’t miss that.

Coming back to our atomic structure model that we are going to be learning about now, it’s a three-dimensional model, as opposed to the two dimensional model that we talked about previously. So it copes with the shape of molecules, the angles of bonding and so on. So it explains the layout of the periodic table and helps us understand how the D-block and the F-block can exist in the periodic table. It also considers the nature of electrons. And for that, we require the quantum mechanics we’ve mentioned and the introduction of quantum numbers, which will help us understand a bit more about why those orbitals exist as well. If you’d like to see the accompanying notes to this episode, which include diagram of the shapes of some of the orbitals and also comparison of the atomic models you’ve learned so far and what you’re going to be learning from this point forward, then go to the show notes that are at chemistrymadesimple.net/2.

In our next episode, we’re going to be defining what quantum mechanics means for chemistry at this stage. We’re also going to make this counterintuitive concept seem simpler, making it more like the world around us that we understand. So what would be good if you subscribe now, join us for that episode and I’ll speak to you then.