Here's why:

* Valence electrons are the electrons that are involved in chemical bonding.

* The outermost energy level of an atom is called the valence shell.

* Electrons in the valence shell are the ones that are furthest from the nucleus and experience the weakest attraction to it. This makes them the most likely to interact with other atoms and form bonds.

Therefore, any electron found in the outer energy level (valence shell) is considered a valence electron.