Valence electrons are those found in the last electronic layer (called orbitals) and are very likely to participate in a chemical reaction. These electrons have the highest n main quantum number.
In the formation of molecules and compounds, they have such importance because they determine whether an atom can create bonds or not. One element binds to another through the valence electrons.
We can also say that these electrons are the ones furthest from the atom’s nucleus, and therefore the ones that can commonly participate in the formation of chemical bonds.
It is the location where the valence electrons are and corresponds to the last level of the atom. In other words, the farthest orbital as you can see in the image below.
There are eighteen vertical columns called Groups and seven horizontal rows called Periods. All elements in the vertical column in a group have the same number of valence electrons (this does not occur in transition metals).
For the main group element, a valence electron is an electron that has the highest main quantum number, n. You can tell the number of valence electrons by looking at the group number. The only exception is Helium, this element is in group 8A, but his valence electron is 2.
In the following paragraphs, we will look for examples with different elements from the periodic table above.