The valence of a chemical element indicates the amount of chemical bonds it makes to be stable.The term ” valence ” started to be used in the study of chemical bonds around the 19th century. Scientists were looking for an explanation for the ability of the elements’ atoms to combine to form substances.
So, as the text Octet rule in chemical bonds shows , scientists Lewis and Kossel proposed an explanation. They noted that the only elements that did not perform chemical bonds and that were found in isolation in nature were noble gases (elements of family 18 or VIII A of the Periodic Table).
Scientists had also discerned that, for example, hydrogen atoms only made one bond, never more than that. On the other hand, oxygen always made two bonds and nitrogen, three bonds.
The elements of the families of these elements made the same number of connections as they did. This showed that the ability to combine the elements was based on empirical rules. What all the noble gases had in common, which the other elements did not have, was that in their last electronic layer there were always eight electrons (with the exception of helium, which has two electrons because it only has one layer (K)).
Then came the electronic valence theory , which said that the atoms of the elements tend to make chemical bonds, losing, receiving or sharing electrons in order to acquire the electronic configuration of noble gases, that is, to have eight electrons in their last layer and thus be stable.
In this way, the term ” valence” came to be used to refer to the combining power that an atom has, that is, the number of bonds that it must make to be stable. For example, if hydrogen only makes one chemical bond, it is monovalent , the oxygen that makes two bonds is divalent and nitrogen is trivalent because it makes three bonds.
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It is very common for people to know that carbon (the basis of Organic Chemistry) is tetravalent , which means that it makes four chemical bonds. That is why there are thousands of organic compounds, as it can make these four bonds with atoms of other elements or with other carbons.
It is obvious to conclude then that what determines the valence of a representative chemical element is the amount of electrons that it already has in its last electronic layer. This is also why this outermost layer is called a valence layer or level.
Note this below:
The following is an example of a chemical element from each family. Observe the valence layer of each:
When the element makes an ionic bond, losing one or more electrons, becoming a cation (positive ion), or gaining electrons and becoming an anion (negative ion), the valence is called electrovalence , being the electrical charge of the ion. For example, sodium has a tendency to make only one bond, so its valence is equal to 1. But when it loses an electron and becomes the Na +1 cation , it is said that its electrovalence is +1.
Some elements, however, have variable valence . An example is phosphorus (P) which can have valences 3 and 5 in different compounds.
