Electron Shells

The idea of electron shells is essential in chemistry and also plays an important part in various areas of physics so needs to be discussed in its own right. But before we look at electron shells let’s be clear that this is just a model – a way of imagining what is going on inside an atom. Nevertheless, it’s a very powerful model that will take you a long way in your science studies, so let’s make a start…

I am assuming that you already know the basics of atoms: if not then make sure you read the introduction that is available in this post. That done, you should know that atoms contain three types of sub-atomic particles; electrons, protons and neutrons. We are only interested here in protons and electrons, which have equal amounts of opposite charge. Protons are given a charge of +1 and electrons have a charge of -1. Atoms of different elements contain different numbers of protons and electrons but atoms always have equal numbers of protons and electrons.

For example, an atom of oxygen contains eight protons and it must also have eight electrons (so that the charges are balanced and cancel out). Iron is a completely different element that has 26 protons and 26 electrons. The numbers of protons and electrons in iron are different from the numbers for oxygen but they are still equal to each other. You may also recall that the number of protons fixes an element’s position in the Periodic Table, and this tells you that oxygen is in position 8 and iron is in position 26.

But how are the electrons arranged in an atom? Do the electrons just “buzz around” in an atom or is there some sort of special arrangement? The answer leads us to the model of electron shells, where we place the electrons in what we can imagine as a series of lanes on a circular running track that goes around the outside of every atom. Each lane is a different electron shell. As the lanes get further away from the centre of the atom, they get bigger so they can hold more electrons. In the diagram below, the centre of the atom is shown as a brown disc and the electron shell “tracks” are dotted circles.

For our purposes, we will limit our analysis to the first 20 elements because this will keep things simple. There is lots of information available online if you want to explore further but I highly recommend that you stick with the first 20 elements for now, as shown below.

You will notice that the columns are headed with Group numbers and the rows have Period numbers. These numbers define the arrangement of the electrons in each element.

You will recall that an element’s position in the Periodic Table (counting from left to right, going top to bottom) gives the total number of protons, which is equal to the number of electrons: the “coordinates” of the element tell you how those electrons are arranged.

The Period of an element tells you how many electron shells are in use and the Group number tells you how many electrons are in the outermost electron shell.

So hydrogen (H) has only one electron shell in use (because it is Period 1) and it has just one electron in its outermost shell (because it is in Group 1). Similarly, carbon (C) has two electron shells in use and has four electrons in its outermost shell. And argon (Ar) has three electron shells in use with eight electrons in the outermost shell. You should locate all three elements in the 20-element version of the Periodic Table, shown above, to check that you agree with the numbers I have just given.

We then need to know one additional fact to complete our electron arrangements: all the inner electron shells that are in use will always be filled to their maximum capacity. This is logical because the last electrons wouldn’t be forced into the outermost shell if there were still room in an inner shell.

The maximum capacities of the shells are;

• Two electrons go in the first electron shell that is closest to the centre of the atom.
• Eight electrons go in the second electron shell.
• Eight* electrons go in the third electron shell.
• Two** electrons go in the fourth electron shell.

If you add the numbers above (2+8+8+2) you will get a total of 20, so that is enough to explain the electron arrangements for the first 20 elements. (This is a simplification: there is an extra note about the truth at the end of this post.)

So, knowing the arrangement of the outermost electrons (using the Period number and the Group number) then adding our knowledge about the maximum capacities of shells that are inside the outermost shell, we can draw the electron arrangements for hydrogen, carbon and argon as shown below. In each case, the outermost shell is shown in red.

If you have understood the concepts explained here then you will now be able to draw the full electron arrangement (electron configuration) for any of the first 20 elements. With that skill, you are well on the way to explaining how elements bond to each other in chemical reactions and therefore the physical properties of different substances that are produced by those reactions.

*/** FOOTNOTE In truth, the third electron shell can actually hold 18 electrons but the fourth electron shell is started once there are eight electrons in the third shell. Similarly, the fourth shell will eventually hold a maximum of 32 electrons but, like in the third shell, the filling takes place in a number of separate stages.