noun, plural: valence electrons
In chemistry and biochemistry, a valence electron pertains to any of the electrons on the outer shell of the nucleus of an atom. It is capable of interacting with other atoms, i.e. by transferring to another atom or by sharing and forming chemical bonds. In a single covalent bond for instance, each of the two atoms shares a valence electron and form a bond.
The valence electrons of an atom accounts for the properties of an element. Based on its valence, it can be determined if the element is capable of bonding with another element and up to what extent, i.e. if it would lose or gain an electron and form an ion or if it would rather share electrons and form bonds.
Valence electron is capable of absorbing and releasing energy (in the form of a photon). The photon is the quantum unit of light energy or electromagnetic radiation. Photons are emitted as electrons move from one energy state to another, and when it is emitted via a biological system, it is referred to as a biophoton. This is what occurs in organisms capable of bioluminescence. In photosynthesis, photons that are absorbed by the chlorophyll reaction centers (e.g. P680) trigger the latter to become an electron donor.