Video Transcript
The diagram shows electrons in different electron shells in an atom. All of the first four electron shells in the atom are filled. Which of the electron shells contains the most electrons? Which of the electron shells contains the fewest electrons?
We can see that in this question we’re given this diagram, and we’re told that it shows an atom with electrons in different electron shells. In this diagram, the red circle at the center represents the atom’s nucleus, and this is where all of the protons and neutrons of the atom are found. Then, surrounding this nucleus, we can see that we have these four black rings. These rings represent the first four electron shells, or energy levels, of the atom. And these shells are occupied by the atom’s electrons. The electrons in a particular shell are represented by these small blue dots drawn on the corresponding ring in the diagram.
We can recall the shell that’s closest to the nucleus, so that’s this one here in our diagram, is known as the first electron shell, or the first energy level. The next shell out from this is the second electron shell. And then we’ve got the third shell and then the fourth one. In principle, there’s then also an infinite number of higher shells that the electrons could occupy. We can recall that in the absence of any external energy that’s capable of exciting the electrons, an atom’s electrons will always occupy the lowest energy level that’s available to them. Since there’s no mention of any kind of external energy in the question, then while all of these higher shells do exist, we wouldn’t expect to find any electrons in them provided the space for those electrons in the first four shells.
Whenever we draw a diagram like this one showing electrons in an atom, we generally only draw the electron shells that have at least one electron in them. In this case, that’s just these first four electron shells of this atom. Now, perhaps we might also wonder if electrons occupy the lowest available energy level, why don’t they all pile up in this first electron shell closest to the nucleus? After all, we can see that there’s just two blue dots representing electrons drawn on this innermost ring. And then there’s a whole load more blue dots on the next ring out, which represents a higher energy level, and more blue dots still on the next ring and the next one, which are both higher energy levels again.
The important thing is that electrons don’t just always occupy the lowest energy level. They occupy the lowest energy level that’s available to them. Each electron shell, or energy level, can only hold a particular number of electrons. And that number is different for different shells. So that’s the reason that the atom’s electrons end up arranged on the electron shells in the way that we can see on this diagram.
Now, let’s turn our attention to what the question is asking of us. We’ve got two parts to this question, and the first part asks which of the electron shells contains the most electrons. And the second part asks which of the electron shells contains the fewest electrons. Now, we’ve already identified that the black rings in the diagram represent the various electron shells and that the blue dots drawn on a particular ring represent the electrons in the corresponding shell. So then to work out which of the electron shells contains the most electrons and which contains the fewest, we just need to identify which of the black rings has the most blue dots on it and which has the fewest blue dots.
Looking at the diagram, it’s obvious to see that that’s the most blue dots on this outermost ring. We know that this ring represents the fourth electron shell or energy level. And so we can say that the electron shell that contains the most electrons is shell four. If we now look again at the diagram, we can see that the ring that’s got the smallest number of blue dots on it is this innermost ring, which has just two blue dots on it. We know that this innermost ring represents the atom’s first electron shell or energy level. And this means that the electron shell that contains the fewest electrons is shell one.
It’s worth briefly mentioning that rather than just looking at the diagram by eye and identifying that the fourth ring clearly has the most blue dots on it, we could have been more thorough and explicitly counted the blue dots drawn on each of the rings. If we did this, we would find that the first ring contains two blue dots, meaning that the atom’s first electron shell contains two electrons. Then on the second ring out, we count eight blue dots, meaning that the second electron shell contains eight electrons. We then count 18 blue dots on the third ring, meaning that the third electron shell has 18 electrons in. Finally, on the fourth ring, we count 32 blue dots, which means that the fourth electron shell has 32 electrons.
With this information about the number of electrons in each of the electron shells, let’s briefly look at another way we could’ve approached this question. We can notice that the question tells us that all of the first four electron shells in the atom are filled. When a given electron shell is filled, it means that that shell contains the maximum number of electrons that that particular shell is capable of holding. It turns out that no matter what atom we’re considering, the first shell can always hold a maximum of two electrons. The second shell can always hold a maximum of eight. The third shell can hold a maximum of 18. And the fourth shell can hold a maximum of 32.
In fact, in general, the higher the electron shell, or energy level, the more electrons that shell can hold. Since we were told that for this atom the first four electron shells are all filled, then we know that each of these four shells contains the maximum number of electrons that shell can hold. Then, since the higher the shell, the more electrons it can hold, we know that the highest shell, which in this case is shell four, must contain the most electrons and the lowest shell, shell one, must contain the fewest electrons.
