The diagram shows electrons in different electron shells in an atom. How many filled shells does the atom have?
Okay, so this diagram is of an atom. And we can see that there’s the nucleus at the center of the atom, which contains the protons and neutrons within the atom. And we can also see electron energy levels represented by these black circles, which contain different numbers of electrons represented by these blue dots. Now, we’ve been asked to find how many filled shells does the atom have.
Well, within the diagram, we can firstly see that there are two shells drawn, where a shell is the same thing as an energy level. The first shell is the inner shell and the second shell is the outer one that’s drawn here. And so, we need to try and work out how many of these two shells are filled. Now, what we mean by filled is that each shell can only hold a certain number of electrons. Let’s recall that the first shell — the inner shell — can only hold up to two electrons, whereas the second shell can hold up to eight electrons. Now, we won’t recall here how many electrons the third shell can hold because we don’t have a third shell in the diagram at all. However, it’s important to remember that the further out the shell from the nucleus or in other words the larger the number of the shell, the more electrons will be able to hold.
But anyway, so coming back to this particular case, we know that the very first shell of an atom can hold up to two electrons. Therefore, if the first shell does indeed hold two electrons, then it’s considered filled. There is no more space for other electrons to occupy that shell. And so, other electrons must occupy higher energy shells. And in this particular case, there are one, two electrons in the first shell. Therefore, the first shell is indeed filled.
And then, we can move on to looking at the second shell. We know that it can hold up to eight electrons. However, in this second shell, there are one, two, three, four, five, six electrons. This means that there’s space for two more electrons in the second shell. And so, the second shell is not filled. So the conclusion that we can arrive to here is that the first shell is filled and the second shell is not filled.
Therefore, the number of shells that are filled is one. And that is the answer to our question.