The Valence, or known as valency, in Chemistry, is the characteristics of an element that indicates the number of other atoms with which an atom of a particular element can form a covalent bond. Introduced in 1868, the term is used for the expression of both the possibility of combination of an element in general and the numerical value of the power of combination. Thus valence meaning is the number of electrons as most of the bonds are formed by the exchange of the valence electrons. The valence electrons determine the valency meaning in Chemistry and define what is valences in Chemistry.
A Brief Account of Valence Chemistry
The explanation and the systematization of the concept of valence or valency was one of the major challenges for 19th-century chemists. As there was no satisfactory theory for the cause of valency, most of the efforts were done on making empirical rules for the determination of the valencies of the elements. The characteristic valence of elements was measured in terms of the number of atoms of hydrogen with which an atom of the element can merge or that it can replace in a compound. It later was quite evident, however, that the valencies of many of the elements varied in different compounds.
The first great step in the development of a satisfactory explanation of valence and chemical combination was made by the American chemist G. N. Lewis in 1916 with the identification of the chemical bond present in the organic compounds as a pair of electrons was held jointly by two atoms which were holding them together. In the same year, the nature of the chemical bond between electrically charged atoms, or also known as ions, was discussed by the German physicist W. Kossel.
The valence theory was reformulated and described incorporating electronic structures and interatomic forces after significant development in the field of electronic theory of periodicity of elements. Due to this several new concepts were introduced which included ionic valence, covalence, oxidation number, coordination number and metallic valence which also explain the different modes of atomic interactions.
Define Valence and Valency
Most of the definitions in Chemistry are defined by IUPAC. Hence, the definitions in valence chemistry are also given by IUPAC. As per the IUPAC valence meaning is defined as the maximum number of single valence atoms that can possibly merge with an atom of the particular element under study, or with a fragment, or for which an atom of this element can be substituted.
Another description used to define valence in modern times is - The number of possible hydrogen atoms that can merge with an element in a binary hydride or twice the number of oxygen atoms that combine with an element in its oxide or oxides forms. This definition differs from the IUPAC definition as an element can be said to have more than one valence possible.
Another modern valence definition stating the valence meaning is that the valence of a particular atom present in a molecule is the number of electrons that an atom uses in the formation of a bond. This is represented by the formulas for calculating the valence electrons that can also be used to define valence is:
Valence = number of electrons in the last/valence shell of a free atom - Number of electrons on an atom in molecules that are not bonded. Hence, valency definition in chemistry can be shown by the formula as
Valence or Valency = Number of bonds formed + Formal charge of the atom.
The Valency and Number of Electrons
The Rutherford Model of the nucleus of an atom shows that the outside of an atom is filled by electrons, suggesting that the electrons are the ones responsible for the interaction of the atoms and the formation of the chemical bonds. Later, G. N. Lewis explained the valence and chemical bonding in terms of the nature of atoms to acquire a stable octet of eight valence-shell electrons. According to the theory proposed by Lewis, covalent bond formation leads to octet structure by sharing of the electrons, while ionic bonding leads to octet structure by the transfer of electrons from one atom to the other.
Another term related to valence is covalence, which is the number of electron pairs shared by one atom with another atom. Also, by name co- means together, indicating that a covalent bond is that bond in which the atoms share a valence. Hence, it is now more common to say covalent bonds rather than using valence and valence meaning in Chemistry in high-level work done in advancing the theory of chemical bonding although it is widely used in the understanding of the basic concepts which provide an introduction to the topic.
Linus Pauling, during the 1930s, proposed the existence of polar covalent bonds which are possible intermediaries in between the covalent and the ionic bond. He also stated that the degree of the ionic character in such a bond was influenced by the electronegativity of the two atoms participating in a bond formation. He also considered the hypervalent molecules. These molecules are the ones in which the main group of elements have valences greater than the maximum of 4 allowed valences by the octet rule.
An example of this case is the sulphur hexafluoride molecule (SF6), where according to Pauling the sulfur forms six two-electron bonds using sp3d2 hybridization of the atomic orbitals, merging one s, three p and two d orbitals. But the recent quantum mechanical calculations on this and similar types of molecules show that the role of the d-orbitals in such bonding is minimal and the SF6 molecule has to be described as having six polar covalent bonds, which are also partly ionic, made from only four of the orbitals on sulfur which is one s and three p following the octet rule, along with six orbitals on the fluorines. Similar calculations on transition-metal molecules also show that the role of p orbitals is minor, and hence one s and five d orbitals on the metal are sufficient enough to describe the bonding.