We explain what valence is in chemistry and what the types of valence are. Also, examples of some chemical elements.
What is valence?
In chemistry, we speak of valence to refer to the number of electrons that an atom of a given chemical element has in its final energy level. Another way to interpret valence is as the number of electrons that an atom of a certain chemical element must give up or accept to complete its final energy level. These electrons are of special relevance, as they are responsible for the formation of chemical bonds, for example, covalent bonds (co-valent: they share valence). It is these electrons that are involved in chemical reactions.
An atom can have one or more valences. For this reason, this concept (created in the 19th century to explain the “affinities” between the different atoms that were known) has been replaced with the “oxidation number”, which ultimately represents practically the same thing.
For example, the hydrogen atom has valence 1, meaning it can share an electron in its last shell; carbon, on the other hand, has a valence of 2 or 4 that is, it can give up two or four electrons. Hence, the valence number represents the element's ability to gain or lose electrons during a reaction or chemical bond.
Throughout history, the concept of valence allowed the development of theories regarding chemical bonds as they are:
- Lewis structure (1916). It is a two-dimensional representation of molecules or ions, where covalent bonds are represented by dashes and unshared electrons by dots. If lone pairs of electrons exist in the structures, they are represented by two points.
- The valence bond theory (1927) This theory states that the central atom in a molecule tends to form pairs of electrons, which depends on geometric limitations of the molecule and compliance with the octet rule (the ions of chemical elements tend to complete their last energy level with 8 electrons to reach a more stable configuration).
- The theory of molecular orbitals (1928) According to this theory, electrons are not assigned to individual bonds between atoms (as proposed in the Lewis structure), but rather these electrons move throughout the molecule under the influence of the atomic nuclei.
- The valence shell electron pair repulsion theory (1958). This theory is based on the electrostatic repulsion of the valence electrons of an atom, which repel each other until reaching an arrangement in space, where they finally no longer repel each other and the geometry of the molecule is defined in this configuration.
Types of valence
There are two different types of valence:
- Maximum positive valence. It reflects the maximum combinatorial capacity of an atom, that is, the greatest number of electrons it can give up. Electrons have a negative charge, so an atom that gives them up obtains a positive valence (+).
- Negative valence. It represents the ability of an atom to combine with another that has a positive valence. Atoms that receive electrons have a negative valence (-).
Valence of the elements
The known valences of some elements of the periodic table are the following:
- Hydrogen (H): 1
- Carbon (C): 2.4
- Sodium (Na): 1
- Potassium (K): 1
- Aluminum (Al): 3
- Mercury (Hg): 1.2
- Calcium (Ca): 2
- Iron (Fe): 2, 3
- Lead (Pb): 2.4
- Chromium (Cr): 2, 3, 6
- Manganese (Mn): 2, 3, 4, 6, 7
- Chlorine (Cl): 1, 3, 5, 7
- Oxygen (O): 1.2
- Sulfur (S): 2, 4, 6
- Nitrogen (N): 1, 2, 3, 4, 5
- Arsenic (As): 3.5
- Boron (B): 3
- Silicon (Si): 4
- Gold (Au): 1, 3
- Silver (Ag): 1
- Phosphorus (P): 3.5
- Radius (Ra): 2
- Magnesium (Mg): 2
- Copper (Cu): 1, 2
References
- «Chemistry» Whitten, Davis, Peck & Stanley. Eighth Edition. CENGAGE Learning, p 251.
- “Chemistry. A project of the ACS» American Chemical Society. Reverté Publishing House (2005). ISBN 9788429170016.
