Valence electrons – all about the topic

While the nucleus determines both the type and weight of the atom, the real magic in chemistry happens in the atomic shell. Here are the so-called electrons, which are responsible for most of the properties of elements and atoms. The electrons in the outermost layer are particularly relevant for this. They are also called valence electrons.

The valence electrons in the atomic model

The best way to understand the importance of these electrons is to start with the structure of an atom. The first model that was designed for it came from Ernest Rutherford and is accordingly called Rutherforsche’s atomic model. As early as 1911 he established that there is the so-called atomic nucleus with the protons and the atomic shell with those same electrons. However, the distribution of the electrons was not described until two years later.

The Bohr atomic model by Niels Bohr is still one of the most important models when it comes to representing atoms, especially electrons.

All electrons with the same energy occupy a so-called shell. Each circular path corresponds to such a shell. The naming started with n=1 in the figure and is then continued from the inside out. Another representation starts with K and then continues alphabetically. K represents the same as n=1. L then stands for n=2 and so on.

You can compare this to an onion that has more than one skin. Each of these shells contains valence electrons, which then determine the properties. For chemical reactions like the ones you find in class, you only look at the outermost shell. Therefore, the valence electrons are also called outer electrons.

Definition of the valence electrons

Before you get more information about the valence electrons, here is a summary of what you have already learned about the outer electrons.

In the next step you can now see how you can easily read the corresponding number of valence electrons in the periodic table.

The valence electrons in the periodic table

The periodic table helps you again to get important information easily. Information about the valence electrons gives you either the main group number or the subgroup number. The first three periods are counted from 1-8, since there are only main groups here. Then count up to 18, because now ten subgroups are added. The group number then corresponds to the number of valence electrons.

In periods 6 and 7 it becomes difficult to state the exact number, since the radioactive lanthanides and actinides are now also added and so theoretically there can be up to 32 outer electrons. Here it is better if you describe the electrons using the electron configuration presented later.

However, the noble gases are of no further relevance for you, since they are very inert and only rarely react at all.

But what does that mean for the bonds that these elements enter into?

Just because an atom has seven outer electrons doesn’t necessarily mean it has to form seven bonds. It can only do so in theory. The best example of this is chlorine, which has those seven valence electrons. It is therefore in the 7th main group and belongs to the halogens.

One of the most well-known bonds that chlorine forms is with sodium. Sodium chloride (NaCl) is formed, which you also know as table salt. In this case it is an ionic bond. Despite the seven valence electrons, there is only one bond to another atom.

With this, however, chlorine achieves exactly what it wants: the inert gas state and thus a low-energy state. The reason for this is the high electronegativity. This represents the attraction of the chlorine atom acting on electrons. At 3.16, chlorine is strong enough to take the electron away from sodium. It therefore gains an additional valence electron, while sodium loses an outer electron.

So far so good. So most of the bonds that chlorine forms look like they do with sodium. However, there are actually cases in which chlorine forms more bonds. All compounds with an odd number of bonds are particularly stable.

In the case of gaseous elements in particular, the valence electrons indicate the maximum number of bonds an element can form. In the case of chlorine, these are the seven bonds that are realized in perchloric acid (HClO4). In reality, however, chlorine usually only forms one bond.

The valence electron configuration

You already got to know the orbital model very briefly at the beginning. This is not only advantageous in that it reproduces the structure of the atoms very realistically. This model makes it possible to specify the distribution of the electrons, the so-called electron configuration. In ascending order of the atomic numbers, only one electron is added at a time. In the distribution, therefore, only one electron needs to be added, the rest can be taken over from the previous element.

The shells presented in this model will not be explained again as this goes far beyond understanding valence electrons. Instead, the following figure gives you a small overview. The periodic table will help you again.

Figure 4: The electron configuration in the periodic table.

If you now want to read the electron configuration, you go along this scheme according to ascending atomic numbers. You will get to know the designation of carbon (C) as an example. This means that carbon with atomic number 6 is right in the box under number 14 in the periodic table.

In order to be able to specify the electron configuration, you start with the 1 and meet the designation 1s there. It stands for an orbital, but as mentioned before, you will learn more about this in the corresponding explanation. You write them down with a small superscript 2, i.e. 1s2, since this orbital can hold a maximum of two electrons. You can also recognize this by the fact that it comprises a maximum of two fields in the periodic table.

After that you continue and go into the second period. The first thing you will find there is 2s. It also includes two squares, so again 2s2. Then it goes further into the area 2p, where there is also carbon. Since only two fields are occupied here, so to speak, only two electrons are added, which is why 2p2 again is written.

In summary, the electron configuration is: 1s2 2s2 2p2.

However, the whole thing can now be abbreviated by the separate valence electron configuration. The last full period is used as the basis and the inert gas is written in square brackets as a symbol. For carbon, the last full period is the first period. The noble gas is correspondingly helium as . Then, in the next period, the electron configuration is only noted for the valence electrons, as is known.

So for carbon the valence electron configuration is: 2s2 2p2.