In chemistry, valence electrons are the electrons containe in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an element reacts chemically with other elements: The fewer valence electrons an atom holds, the less stable it becomes and the more likely it is to react. The reverse is also true, the more full/complete the valence shell is with valence electrons, the more inert an atom is and the less likely it is to chemically react with other chemical elements or with chemical elements of its own type. This is because it takes more transfer of energy(photons) to lose or gain an electron from or into a shell when that shell is more complete/full.