An electron in the outermost energy level of an atom is called a valence electron.

Here's why:

* Valence electrons are the electrons that participate in chemical bonding. They are the ones most likely to be involved in interactions with other atoms.

* Outermost energy level: The outermost energy level, also known as the valence shell, is the highest energy level occupied by electrons in an atom.

* Chemical behavior: The number of valence electrons an atom has largely determines its chemical properties and how it will bond with other atoms.

Let me know if you'd like to know more about valence electrons or chemical bonding!