# Question Video: Identifying the Number of Filled Electron Shells in an Atom Physics

The diagram shows electrons in different electron shells in an atom. How many filled shells does the atom have?

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### Video Transcript

The diagram shows electrons in different electron shells in an atom. How many filled shells does the atom have?

Okay, so we’ve been given this diagram, and we’re told that it shows an atom with electrons in different electron shells. In the center of the diagram, we’ve got this red circle here, and this represents the nucleus of the atom, which we can recall contains all of the atoms, protons, and neutrons. Then around this red circle or nucleus, we can see that we’ve got these three black rings. These represent three different electron shells, which are shells or energy levels that the atom’s electrons can occupy. These electrons are represented in this diagram by these blue circles that are drawn on all of the different shells.

The electron shell that’s closest to the nucleus is labeled as the first energy level. The next one is the second energy level. And the next one is the third. In fact, while this diagram only shows three different electron shells, there are actually an infinite number getting further and further away from the nucleus. The reason that we usually only draw the first few electron shells when we draw a sketch of an atom is that electrons occupy the lowest energy level that’s available to them. So although in principle shell number 56 and shell number 981, for example, both exist, we wouldn’t actually expect to find any electrons in either of these shells.

The other thing that we might wonder is, “If electrons always occupy the lowest energy level that’s available, then why don’t they all pile up in this lowest energy level, that, is the innermost electron shell? After all, we can clearly see from this sketch that there are only two electrons on this innermost ring. And then there’s a whole load more electrons on the next ring out, which is a higher energy level. And there’s even some electrons on the third ring, which is a higher energy level still.”

Well, the important thing to notice is this word here, available. It’s not just the lowest energy level; it’s the lowest energy level that’s available for that electron. It turns out that each of these electron shells or energy levels can only hold a certain number of electrons. The number of electrons that a particular shell can hold is different for different shells. The first shell or energy level, so that’s the one nearest to the nucleus, can hold a maximum number of two electrons. Any further electrons will then have to go into higher shells. The second shell, so that’s the next one out, can hold a maximum number of eight electrons. And then any further electrons will have to go into the third shell, which can hold a maximum number of 18.

Now in this question, we’re asked how many filled shells this atom has. Let’s recall that an electron shell is considered filled when it contains the maximum number of electrons that that particular shell can hold. So for example, we would say that the first shell is filled if it contains two electrons. However, if the second shell contained two electrons, then it wouldn’t be a filled shell because the second shell can hold a maximum number of eight electrons. So we would only say that the second shell is a filled shell if it’s got eight electrons in. Then, in just the same way, the third shell is only a filled shell if it contains 18 electrons.

With all this in mind, let’s now take a look at the electrons in each of the electron shells in the diagram that we’ve been given. If we begin with the first shell, so that’s this one nearest to the nucleus, then we can clearly see that there are two electrons in this shell. That means that we can say that for this atom shell number one is a filled shell. Now let’s look at the second shell. So that’s this next one out from the nucleus. We can number off the electrons starting with the one at the top and then working our way around the circle in a clockwise direction. When we’ve counted them all up, we find that in total there are eight of them. And since we know that eight is the maximum number of electrons that this second shell can hold, then we can safely say that the second shell of this atom is also filled.

The next shell out again is the third electron shell. And we can see that this just has this electron here and this one here, so that’s two electrons in total. Since this third shell is capable of holding a maximum of 18 electrons, then the third shell of this atom is not filled. Since we’ve already said that electrons will occupy the lowest shell or energy level that’s available to them, then if the third shell is not filled, like in this case here, we can safely say that the fourth shell, the fifth shell, and any other higher shells also will not be filled. In other words, for this atom, the only filled shells are the first and second electron shells. That makes two filled shells in total. And so our answer to this question is that the number of filled shells that this atom has is two.