Video Transcript
The diagram shows electrons in different electron shells in an atom. The outermost shell is unfilled. How many more electrons can the atom have in its outermost shell?
Okay, so we’ve been given this diagram, and we’re told that it shows an atom with electrons in different electron shells. The red circle at the center of the diagram represents the nucleus of the atom, which we can recall contains all of the atom’s protons and neutrons. Then, surrounding this nucleus, we can see that we’ve got these three black rings. Each of these rings represents one of the atom’s electron shells, which are also known as energy levels. These are the shells that the atom’s electrons can occupy. Each one of these small blue circles that’s drawn on one of the black rings in this diagram represents an electron that’s in that particular electron shell or energy level.
The shell that’s closest to the nucleus, which in our diagram is this one here, is known as the first electron shell or the first energy level. The next one out is the second electron shell or energy level, and then we’ve got the third shell, and so on. Now, in principle, there’s actually an infinite number of further shells that the electrons of the atom can occupy. As we get further and further out from the nucleus going to higher and higher electron shells, the energy of those shells increases.
It’s worth mentioning that when we talk about the energy of an electron shell, what we really mean is the energy of an electron that’s in that shell. So, electrons in the first shell or energy level will have the lowest value of energy. Electrons in the second shell will have more energy than this. Then, electrons in the third shell will have even more energy again and so on.
Now, this diagram only shows the first three electron shells, despite the fact that, in principle, there’s an infinite number of higher shells. The reason for this is that when we draw a diagram of an atom, we generally only draw the electron shells that have at least one electron in them. We can recall that electrons will always occupy the lowest energy level that’s available to them.
The question tells us that this atom’s outermost shell, which we can see from the diagram is the third electron shell, is unfilled. The fact that this third shell is unfilled means that it has room to hold more electrons than it currently has. Then, since we know that electrons will occupy the lowest available energy level and we know that this atom’s third shell has space to hold more electrons, we can safely say that there won’t be any electrons in the fourth shell, the fifth shell, or any other higher shell.
Now, the other thing we might wonder is if electrons will always occupy the lowest available energy level, then why don’t they all pile up in this first electron shell? After all, we can see from the diagram that there are only two electrons drawn on this innermost ring. And then there’s a whole load more electrons on the next ring out, which represents a higher energy level. And then there’s more on the next ring out again, which is a higher energy level still. It turns out that the key thing here is this word available. Electrons don’t just occupy the lowest energy level. They occupy the lowest energy level that’s available for them. Each electron shell or energy level can only hold a particular number of electrons, and that number is different for each different shell.
Since in this question electrons only occupy the first three electron shells or energy levels of the atom, let’s just consider these first three shells. The first shell, so that’s the shell that’s nearest to the nucleus, can hold a maximum number of two electrons. When this first shell has two electrons in it, we say that the shell is filled because it can’t hold any more electrons. Any further electrons will have to go into the second shell. This second shell can hold a maximum number of eight electrons. And then when it’s got eight electrons in, the shell is filled and any further electrons will go into the third shell. Then, the maximum number of electrons that this third shell can hold is equal to 18.
In this question, we are being asked how many more electrons the atom can have in its outermost shell. Now, we know that the third electron shell is the outermost shell. And we also know that this third shell can hold a maximum number of 18 electrons, so let’s see how many electrons this third shell already has. To count these electrons, let’s number off the blue circles that are drawn on this outer ring, working our way around the circle in the clockwise direction. When we do this, we find that in total there are eight of them, which means that there’re eight electrons in this third shell.
If we take the maximum capacity or the maximum number of electrons that a given shell can hold and we subtract from that the number of electrons that are currently in that shell, then this will give us the number of spaces that that shell has for more electrons. In this case, we are talking about the third electron shell, and we know that the maximum capacity of this shell is equal to 18 electrons. We have also counted that for the atom in this question, the current number of electrons in this third shell is equal to eight.
So then, we can say that the number of extra electrons that this atom has room for in its third or outermost shell is equal to 18 minus eight. This difference works out as 10. So, our answer is that this atom can have 10 more electrons in its outermost shell.
