InstructorPriyanka Mishra
TypeOnline Course
DateSep 3, 2016
PriceFree
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In chemistry, the valence or valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. The concept of valence was developed in the second half of the 19th century and was successful in explaining the molecular structure of inorganic and organic compounds. The quest for the underlying causes of valence led to the modern theories of chemical bonding, including the cubical atom (1902), Lewis structures (1916), valence bond theory (1927), molecular orbitals (1928), valence shell electron pair repulsion theory (1958), and all of the advanced methods of quantum chemistry.

The combining power or affinity of an atom of an element was determined by the number of hydrogen atoms that it combined 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, so phosphorus has a valence of 5 in phosphorus pentachloride, PCl5. Valence diagrams of a compound represent the connectivity of the elements, with lines drawn between two elements, sometimes called bonds, representing a saturated valency for each element. Examples are:

Compound H2 CH4 C3H8 C2H2 NH3 NaCN H2S H2SO4 Cl2O7
Diagram Wasserstoff.svg Methane-2D-flat-small.png Propane Lewis.svg Acetylene-2D.png Ammoniak.png Sodium cyanide-2D.svg Hydrogen sulfide.svg Sulfuric acid chemical structure.png Dichlorine heptoxide.svg
Valencies Hydrogen, 1 Carbon, 4
Hydrogen, 1
Carbon, 4
Hydrogen, 1
Carbon, 4
Hydrogen, 1
Nitrogen, 3
Hydrogen, 1
Sodium, 1
Carbon, 4
Nitrogen, 3
Sulfur, 2
Hydrogen, 1
Sulfur, 6
Oxygen, 2
Hydrogen, 1
Chlorine, 7
Oxygen, 2

Valence only describes connectivity; it does not describe the geometry of molecular compounds, or what are now known to be ionic compounds or giant covalent structures. A line between atoms does not represent a pair of electrons as it does in Lewis diagrams.

Section 1Modern definitionsFree Preview

Valence is defined by the IUPAC as:

The maximum number of univalent atoms (originally hydrogen or chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted.

An alternative modern description is:

The number of hydrogen atoms that can combine with an element in a binary hydride or twice the number of oxygen atoms combining with an element in its oxide or oxides.

This definition differs from the IUPAC definition as an element can be said to have more than one valence.

Section 2Chemical Bonding