The valence electrons of an element are those that can be used to form chemical bonds. Materials that have valence electrons are good conductors of electrical current.
Metals like copper have a few valence electrons, while most nonmetals do not. They can either share electrons (covalent bond) or remove electrons from a neighboring atom (ionic bond).
Most transition metals have two valence electrons. They are the electrons in the highest energy level of the principal quantum number n, or 2nd sublevel. These electrons are the most energetically accessible to accepting electrons to form chemical bonds.
In general, transition metals can have up to a maximum of 6 valence electrons, but there are exceptions. Scandium, for example, has three valence electrons. This is because the element can achieve a +3 oxidation state in ScCl3, but not a +4 or higher oxidation state.
Chromium, on the other hand, has six valence electrons. It can achieve a +6 oxidation state in Cr2O7, or a +4 or higher oxidation states in CrO2-4.
Helium is another exception, because it has a 1s2 configuration with two valence electrons. However, despite this, it is chemically inert and placed in group 18 along with the other noble gases.
The valence electrons of an element determine whether it is a metal or insulator. If a solid compound containing the element is a metal, the valence electrons of the metal atoms are used to form ionic bonds with other atoms to form the molecule. If the metal atoms are not used to form ionic bonds with other elements, they are usually classified as insulators.
