Valence electrons: definition in chemistry
Valence electrons: The electrons in the outermost shell of an atom; they are the ones that take part in chemical bonding.
The number of valence electrons largely decides how an element reacts, because atoms tend to gain, lose or share electrons to reach a stable arrangement, often eight outer electrons (the octet rule). Lewis dot structures show valence electrons as dots around the element symbol. For transition metals the picture is more complicated, because electrons in the inner d sublevel can also take part in bonding.
Examples
- Sodium has 1 valence electron and loses it to form Na⁺
- Carbon has 4 valence electrons and usually forms four bonds
- Oxygen has 6 valence electrons and forms two bonds in water
- Neon has 8 valence electrons, a full outer shell
On the periodic table
For main-group elements the group tells you the number of valence electrons: groups 1 and 2 have 1 and 2, and groups 13 to 18 have 3 to 8 (helium has 2).