In chemistry , the valence US spelling or valency British spelling of an atom is a measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Valence is generally understood to be the number of chemical bonds that each atom of a given chemical element typically forms.
Double bonds are considered to be two bonds, triple bonds to be three, quadruple bonds to be four, quintuple bonds to be five and sextuple bonds to be six. In most compounds, the valence of hydrogen is 1, of oxygen is 2, of nitrogen is 3, and of carbon is 4. Valence is not to be confused with the related concepts of the coordination number , the oxidation state , or the number of valence electrons for a given atom. The valence is the combining capacity of an atom of a given element, determined by the number of hydrogen atoms that it combines with.
In methane , carbon has a valence of 4; in ammonia , nitrogen has a valence of 3; in water, oxygen has a valence of 2; and in hydrogen chloride, chlorine has a valence of 1.
Chlorine, as it has a valence of one, can be substituted for hydrogen in many compounds. Phosphorus has a valence 3 in phosphine PH 3 and a valence of 5 in phosphorus pentachloride PCl 5 , which shows that an element may exhibit more than one valence. The structural formula of a compound represents the connectivity of the atoms, with lines drawn between two atoms to represent bonds.
An alternative modern description is: [ 3 ]. This definition differs from the IUPAC definition as an element can be said to have more than one valence.
