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?
So we’ve got this diagram here and we’re told that it shows an atom with electrons in different electron shells. This red circle at the center of the diagram represents the nucleus of the atom. So that’s where all of the atoms, protons, and neutrons are found. Then surrounding this nucleus, we’ve got these two black rings, each of which represents one of the atom’s electron shells. These electron shells, which are also known as energy levels, are the shells that the electrons of the atom can occupy. 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. Then, the next shell out from this is known as the second shell or energy level.
Now in principle, there are actually an infinite number of further shells that the electrons of the atom can also occupy. As we get further and further from the nucleus getting to higher electron shells, the energy of those shells increases. When we talk about the energy of an electron shell, what we really mean is the energy of an electron that occupies that particular shell. So electrons in the first shell or energy level will have the lowest value of energy, and then electrons in the second shell will have more energy than this. Electrons in the third shell would then have even more energy again and so on for higher shells.
In our diagram, the electrons in each of the first two energy levels are drawn as the small blue circles on each of the two black rings. We said that an atom has an infinite number of electron shells, but we can see that this diagram shows only two 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 unless excited by some external energy electrons will always occupy the lowest energy level that’s available to them. The question tells us that the outermost shell of this atom is unfilled. The outermost shell is the second one, and the fact that it’s unfilled means that it has room to hold more electrons than it currently does.
Since there’s no mention of any external energy source in this question, we know that any electrons in this atom will occupy the lowest available energy level. Since we also know that this atom’s second shell has space to hold more electrons, this means we know that there won’t be any electrons in the third shell, the fourth shell, or any other higher shell.
Now, the other thing we might wonder is if electrons occupy the lowest available energy level, why don’t they all pile up in this first electron shell? We can see from the diagram 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 represents a higher energy level. It turns out that this word here “available” is actually really important. 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 certain number of electrons. The number of electrons that a particular shell can hold is different for different shells.
Since in this question electrons only occupy the first two electron shells or energy levels of this atom, we’ll just consider these first two shells. The first shell, so that’s the shell 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. In the atom shown in this question, this second shell is the highest shell that has electrons in. That makes it the outermost shell of this atom.
The question asks us how many more electrons the atom can have in this outermost shell. Since we know that the maximum number of electrons this second shell can hold is equal to eight, then we just need to count up the number of electrons we’ve got in that shell currently and subtract this number from eight to find out how many more electrons there are room for. To count the electrons in the atom’s second electron shell, let’s number off the blue circles on this outer ring working our way clockwise around the circle.
When we do this, we find that in total there are seven of them, which means that there are seven electrons in the second electron shell. Then the number of spaces to have more electrons in a particular electron shell is equal to the maximum capacity of that shell, so the maximum number of electrons that the shell can hold minus the number of electrons that are in that shell already.
For the second or outermost shell of the atom shown in this diagram, we know that the maximum number of electrons the shell can hold is equal to eight, while the current number of electrons in that shell is equal to seven. So then the amount of extra electrons that this atom can have in its outermost shell is equal to eight minus seven. This difference works out as one. So our answer is that the atom can have one more electron in its outermost shell.