We explain what valence is in chemistry and what are the types of valence. Also, examples of some chemical elements.
What is valencia?
In chemistry, we speak of valence to refer to the number of electrons that an atom of a given chemical element has at its last level of Energy. Another way of interpreting valence is as the number of electrons that an atom of a certain chemical element must give up or accept to complete its last energy level. These electrons are of special relevance, since they are responsible for the formation of chemical bonds, for example, covalent bonds (co-valent: they share valence). It is these electrons that intervene in the chemical reactions.
An atom can have one or more valences. For this reason this concept (created in the nineteenth century to explain the "affinities" between the different atoms known) has been replaced with that of "oxidation number", which finally represents practically the same thing.
For example, the hydrogen atom has valence 1, which means that 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 ability of the element to gain or give up electrons during a reaction or Chemical bond.
Throughout history, the concept of valence allowed the development of theories regarding chemical bonds, such as:
- Lewis structure. It is a two-dimensional representation of the molecules or the ions, where covalent bonds are represented by dashes and unshared electrons by dots. If there are lone electron pairs in the structures, they are represented by two points.
- The theory of the valence bond. 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 the chemical elements have to complete their last energy level with 8 electrons to reach a more stable configuration).
- The theory of molecular orbitals. According to this theory, electrons are not assigned to individual bonds between atoms (as stated in the Lewis structure), but rather these electrons move throughout the molecule under the influence of atomic nuclei.
- The electron pair repulsion theory of the valence shell. This theory is based on the electrostatic repulsion of the valence electrons of an atom, which mutually repel each other until reaching an arrangement in space, where finally they 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 largest number of electrons it can give up. Electrons are negatively charged, so an atom that gives them up gets a positive valence (+).
- Negative Valencia. Represents the ability of an atom to combine with another that has a positive valence. Atoms that receive electrons have a negative valence (-).
Valencia of the elements
The known valences of some elements of the periodic table are as follows:
- 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