"The outermost electrons of an atom that participate in chemical bonding."

Here's why:

* Outermost: Valence electrons reside in the highest energy level (the outermost shell) of an atom.

* Chemical bonding: They are the electrons that are most likely to be involved in forming chemical bonds with other atoms. This is because they experience the weakest attraction to the nucleus and are therefore more easily shared or transferred.

Other phrases that are also accurate but less comprehensive include:

* "Electrons responsible for an element's chemical properties."

* "Electrons involved in chemical reactions."

Let me know if you'd like to learn more about valence electrons and their role in chemistry!