Valency electrons

The electrons of the various sublevels do not differ from each other, exept in the amount of energy. Also the electrons in the outer shell (the valency electrons) are not of a different type than the other electrons in the atom. Yet we gave them this special name 'valency electrons' because of their special function. They define in an important way the properties of the element.
Normally the main shells have a preference number of electrons. When they are satisfied, the electron level is stable. The outer main level always (!) had a preference for 8 electrons, but in reality, only the noble gases have this preference numberr in their outer shell.
All the other atoms do not have such stable outer shell, and so they will always try to stablilize their situation. They do so by cooperating with other atoms. Toghether they can have succes.
Such a process to work toghether and arrive at a more stable situation is calles a 'chemical reaction'. (see also module 07)
So: The valency electrons play a main role in the creation of chemical bonds (see also module 03) during a chemical reaction (see module 07).

The electronic structures of the noble gases are, in a certain way, similar.
Helium has not 8 valency electrons, but still is a noble gas.