An electron is a negatively charged subatomic particle that orbits around the nucleus of an atom. The negative charge of the electrons is balanced by the positive charge of the protons in the nucleus. Electrons have a much lower mass than protons.
The activity of valence electrons helps determine the characteristics of an element. Valence electrons are the electrons orbiting in the outermost energy level of the atom. Elements that have the maximum of eight electrons in their outermost energy levels are the most stable; these elements belong to the family of noble gases. Elements that don't have this full complement of electrons in their outermost energy levels are not as stable and want to form compounds with other elements to attain this stable configuration. The most reactive elements are those that have one or seven electrons in their valence shells; those with one valence electron readily shed their single electrons, whereas elements with seven electrons need to gain a single electron to achieve stability.