Valence electrons

Valence electrons are electrons of an atom which have the highest possible energy level. In the Bohr model of the Atom this reads: Which are on the most elevated shell of the atom. 

The maximum of Valence electrons is 8 because this is the maximum of spare positions on the highest level. Atoms with exactly 8 Valence electrons are inert. They do not form molecules neither with the atoms of the same chemical element nor with other elements. 

As all electrons tend to reach the lowest energy level possible they rather try to find valence positions in the atomic shell of another atom than to remain on an elevated valence level. Consequence is that nature has a strong tendency to occupy completely the valence level of all atoms. The mechanism resulting is, that atoms with 1 up to 4 valence electrons look for partners with 1 up to 4 spare valence positions to form a team with completely occupied valence level together. This is the process of forming chemical bindings or molecules. Stable molecules have their valence levels completely filled with electrons. 

In a situation a molecule has not exactly the valence shell occupied with electrons it is a radical. A radical has a strong tendency to find another radical to form a stable molecule. That means that radicals are very reactive.

In a plasma molecules and atoms are stimulated by energy imput in such an extent, that valence electrons are torn from their stable positions. This process may have theree possible consequences:

stimulated atoms and molecules: One or more valence electrons are elevated to a higher energy level

ions: One or more valence electrons have been completely torn out from the electron shell. The electron has become a free electron and the atom or molecule has become a positive charged ion.

radicals: Molecules are separated into two or more radicals

chemical binding
Molecules are chemical bindings where the valence electrons of the partners form completed valence levels in total.