Valence Definition in Chemistry

What Is Valence in Chemistry?

Valence is a measure of outer shell electrons.
Valence is a measure of outer shell electrons. Science Photo Library - MEHAU KULYK, Getty Images

Valence Definition

Valence is typically, the number of electrons needed to fill the outermost shell of an atom. Because exceptions exist, the more general definition of valence is the number of electrons with which a given atom generally bonds or number of bonds an atom forms. (Think iron, which may have a valence of 2 or a valence of 3.)

The IUPAC formal definition of valence is the maximum number of univalent atoms that may combine with an atom.

Usually, the definition is based on the maximum number of either hydrogen atom or chlorine atoms. Note the IUPAC only defines a single valence value (the maximum), while atoms are known to be capable of displaying more than one valence. For example, copper commonly carries a valence of 1 or 2.

Examples: A neutral carbon atom has 6 electrons, with an electron shell configuration of 1s22s22p2. Carbon has a valence of 4, since 4 electrons can be accepted to fill the 2p orbital.

Common Valences

Atoms of elements in the main group of the periodic table may display a valence between 1 and 7 (since 8 is a complete octet).

  • Group 1 (I) - Usually displays a valence of 1. Example: Na in NaCl
  • Group 2 (II) - Typical valence is 2. Example: Mg in MgCl2
  • Group 13 (III) - Usual valence is 3. Example: Al in AlCl3
  • Group 14 (IV) - Usual valence is 4. Example: C in CO (double bond) or CH4 (single bonds)
  • Group 15 (V) - Usual valences are 3 and 5. Examples are N in NH3 and P in PCl5
  • Group 16 (VI) - Typical valences are 2 and 6. Example: O in H2O
  • Group 17 (VII) - Usual valences are 1 and 7. Examples: Cl in HCl

Valence vs Oxidation State

There are two problems with "valence". First, the definition is ambiguous. Second, it's just a whole number, without a sign to give you any indication of whether an atom will gain an electron or lose its outermost one(s).

For example, the valence of both hydrogen and chlorine is 1, yet hydrogen usually loses its electron to become H+, while chlorine usually gains an additional electron to become Cl-.

The oxidation state is a better indicator of the electronic state of an atom because it has both magnitude and sign. Also, it's understood an element's atoms may display different oxidation states depending on the conditions. The sign is positive for electropositive atoms and negative for electronegative atoms. The most common oxidation state of hydrogen is +8. The most common oxidation state for chlorine is -1.

Brief History

The word "valence" was described in 1425 from the Latin word valentia, which means strength or capacity. The concept of valence was developed in the second half of the 19th century to explain chemical bonding and molecular structure. The theory of chemical valences was proposed in an 1852 paper by Edward Frankland.