Valence electrons

Valence electrons are also called outer electrons. This is because they are located on the outermost electron shell of an atom.

According to Bohr’s atomic model, the atomic nucleus is surrounded by several electron shells. To be more precise, according to the shell model, there are four electron shells called K, L, M, and N shells. The valence electrons are the electrons that are on the outer shell. This is why they are also called valence shells. If there is a chemical bond between two or more atoms, you can imagine that their outer electrons combine with each other. Of course, it depends on how many electrons are on the outer shell.

Each element has a certain number of electrons, which are distributed in a certain way on the four shells. For each shell there is a maximum number of electrons that can be in it. How many electrons that is per shell is shown in this table:

Shell Number of electrons per shell
K 2
L 8
M 18
N 32

Example: Oxygen

Oxygen has atomic number 8 in the periodic table. The atomic number tells you how many protons are in the nucleus of oxygen.
If oxygen is externally neutral, i.e. not charged, the atomic number also corresponds to the number of electrons orbiting on the electron shells of oxygen. This is the case in our example. Therefore, we can now distribute eight electrons among the shells.
The shells are always occupied from the inside to the outside. There is room for two electrons in the K shell. So from the eight electrons there are now six electrons left. Eight electrons fit into the L-shell.
Therefore you find the remaining six electrons in the L-shell.
So oxygen has six valence electrons. In this example you call this shell the valence shell.

In the periodic table you can see immediately that oxygen has 6 valence electrons. For this you have to look up in which main group oxygen is, because the number of the main group corresponds exactly to the number of valence electrons. Oxygen is in the sixth main group and therefore also has six outer electrons.

This rule applies to all main group elements. The only exception is helium.
It is in the 8th main group, but it has only 2 valence electrons. This is because it has a total of only 2 electrons. Valence electrons are important for the formation of bonds between two or more atoms. When valence electrons are involved in a bond, they are also called bond electrons.

An element can theoretically form as many bonds as it has valence electrons, but it can also form fewer bonds.
So the number of valence electrons does not necessarily correspond to the number of bond electrons.

Example: The element chlorine. Chlorine is in the seventh main group and thus has seven outer electrons. If it occurs in the molecule perchloric acid, then all seven outer electrons are actually also involved in a bond. In this case, the number of valence electrons corresponds to the number of bond electrons.

However, chlorine can also form a bond with hydrogen, for example they can combine to form hydrochloric acid. Then only one of the 7 outer electrons is involved in the bond. The number of bonding electrons is therefore 1. Here a so-called electron pair bond is formed. Both atoms share the electrons of their bond, so to speak. You can then say that chlorine has eight valence electrons instead of seven, which leads to a stable state. You can justify this with the octet rule.