In chemistry, valence electrons are the electrons contained 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 number of valence electrons
|Helium atom (not to scale)|
|Helium atom model|
This helium (He) model displays two valence electrons
located in its outermost energy level.
Helium is a member of the noble gases and contains
two protons, neutrons, and electrons.
The number of valence electrons of an element is determined by its periodic table group (vertical column) in which the sup element is categorized. With the exception of groups 3–12 (transition metals), the number within the unit's place identifies how many valence electrons are contained within the elements listed under that particular column.
|Periodic table group||Valence electrons|
|Group 1 (I) (alkali metals)||1|
|Group 2 (II) (alkaline earth metals)||2|
|Groups 3-12 (transition metals)||1 or 2*|
|Group 13 (III) (boron group)||3|
|Group 14 (IV) (carbon group)||4|
|Group 15 (V) (nitrogen group)||5|
|Group 16 (VI) (chalcogens)||6|
|Group 17 (VII) (halogens)||7|
|Group 18 (VIII or 0) (noble gases)||8**|
* The count of valence electrons is not generally useful for transition metals.
** Except for helium, which has only two valence electrons.
Valence electrons in chemical reactions
The number of electrons in an atom's outermost valence shell governs its bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the periodic table of the elements. As a general rule, the fewer electrons in an atom's valence shell, the more reactive it is. Group 1 alkali metals are therefore very reactive, with lithium, sodium, and potassium being the most reactive of all metals.
Every atom is much more stable, or less reactive, with a full valence shell. This can be achieved one of two ways: an atom can either share electrons with neighboring atoms, a covalent bond, or it can remove electrons from other atoms, an ionic bond. Another form of ionic bonding involves an atom giving some of its electrons to another atom; this also works because it can end up with a full valence by giving up its entire outer shell. By moving electrons, the two atoms become linked. This is known as chemical bonding and serves to build atoms into molecules or ionic compounds. Five major types of bonds exist:
- ionic bonds
- covalent bonds
- coordinate covalent bonds (also called dative covalent bonds)
- hydrogen bonds
- metallic bonds
The valence electrons are also responsible for determining the electrical conductivity nature of an element.