In chemistry, valence (also known as valency) is used to measure how powerful an element is in combining molecules or chemical compounds.
The concept was proposed in 1852 by Sir Edward Frankland, who sought to explain why atoms and molecules combine in certain ways. It was later discovered that the valency of an element is directly related to the positions of electrons in the outermost shell of atoms.
Continue reading to learn more about the concept of valence, how it was invented, and its impact on modern chemistry.
In this post:
The history of valence
Since ancient times, alchemists have observed that elements and compounds react preferentially and proportionately with other substances. This can be measured by comparing the weights of the reactants and products, as well as the unreacted excess substances.
Conversely, when substances are reduced into simpler constituents, the resulting products can weigh less than the original substance. For instance, the ashes created by burning wood are lighter than the wood itself.
However, the concept of valence only really began to emerge when the English chemist, Sir Edward Frankland, started experimenting with isolating organic radicals. This led to his discovery of organometallic compounds.
Frankland realised organometallic compounds could be formed via the substitution of organic groups for oxygen, sulphur, chlorine, and other elements. He theorised that each element can only combine with a limited number of atoms of other elements.
In other words, there’s a saturation point of chemical reactions for each element. In 1852 he published his pioneering paper on the theory of valency, which served as the foundation for other analytical discoveries in chemistry.
Early understanding of atomic structure
The earliest known conception of atoms is believed to date back to the Greek philosopher Democritus in about 430 BCE. He conceived the idea of atoms as the indivisible and indestructible basic units of matter, a belief that lasted for around two thousand years.
In the 1800s, experimental physics revealed that atoms are composed of subatomic particles. John Dalton expounded on the philosophical concept of the atom and turned it into a scientific theory.
He developed the idea of the law of definite proportions. This theory states that whenever two elements combine to form more than one compound, the weight of one element in one of the products is a simple integer. Elements form simple integer ratios when forming compounds.
The valence theory
Valence is a concept in chemistry that states each element has a maximum limit of other elements or molecules with which it can combine. This limit can be represented by a numerical value.
Here are some of the key figures who helped shape our understanding of valence and chemical structure:
Edward Frankland – the pioneer
Long before electrons were discovered and the Lewis concept of electron-pair bond was established, Edward Frankland developed the theory of valency.
In 1852, the British chemist proposed the theory of chemical valency. It was Frankland’s earlier work on organometallic compounds that led him to the theory of combining power and subsequently valency.
His understanding was based on the definite saturation capacity of each element to react with other elements and compounds. This was the basis of modern structural chemistry, with Edward Frankland as its pioneer.
August Kekulé
The German organic chemist August Kekulé is widely known for formulating the theory of chemical structure.
This theory was based on the idea of atomic valence, particularly on the tetravalence of carbon. He also expounded on the chemical properties of carbon atoms to form chains. He was able to deduce the bonding order of atoms in a molecule.
Archibald Scott Couper
Couper was a Scottish chemist who – independently of Kekulé – proposed the idea of the tetravalency of carbon and its ability to link to other carbon atoms to form chains of molecules.
However, at the time, Couper’s ground-breaking work went largely unnoticed and ill health brought his career to an untimely end. It wasn’t until years after his death that his scientific achievements were given the recognition they deserved.
Defining valence in modern chemistry
Before the discovery of subatomic particles and the structure of the atom, the concept of valence as applied in chemical reactions focused on the fixed proportionality of reactants and products. However, the definition of valence in modern chemistry is linked to the energy levels or shells of electrons.
Valence electrons are the electrons found in the outer shells of atoms. They’re directly involved in chemical bonding as they pair up, either covalently or ionically, with other electrons.
Valence electrons follow the octet rule, or the tendency of atoms to have eight electrons in the outer valence shells. Elements with eight electrons in the valence shells (the noble gases, for example) are inert, while those with one or two electrons tend to be highly reactive.
In modern chemistry, valency is key to predicting and explaining chemical reactions. As a measure of combining power, it can predict the hierarchy of reactions given a compound and an element. It can also indicate whether certain elements will be able to substitute other elements.
The impact of valence theory on chemistry
The valence theory has had a significant impact on modern chemistry, providing the framework for much of what we know about how chemical reactions occur and how molecules are structurally arranged.
We can use the valence theory to predict which elements and compounds will react with each other, as well as which products are likely to be formed. More importantly, we can also use our knowledge of valency to predict the structure of molecules and their chemical properties (to a certain extent, at least).
Thanks to our understanding of valency, it’s now possible to design molecules to human advantage. This has enabled advancements in many different fields, with scientists now able to create all manner of drugs, chemicals, materials, and weapons based on their knowledge of atomic valency.
Advancements in organic chemistry
From the initial experiments of Sir Edward Frankland to modern drugs, organic chemistry has come a long way.
Among the scientific fields to benefit from these developments are analytical chemistry, industrial chemistry, forensics, the pharmaceutical industry, medicine, and environmental science.
Some of the notable advancements in organic chemistry include C-H activation, asymmetric synthesis involving catalysts, flow chemistry, automated synthesis, and green chemistry.
Valence in modern chemical applications
Our knowledge of atomic valency and the predictability of reactions means we’re now able to synthesise various organic compounds, as well as biomolecules such as enzymes and proteins. We can even isolate functional groups and mimic organic molecules like the scents and tastes of fruits.
The concept of atomic valency also comes in useful when analysing unknown compounds. For example, certain reagents can be added to analytes to test if displacements occur. From this, we can deduce what types of elements, or at least groups of atoms, are present in a compound, solution or mixture.
Relevance in chemical manufacturing
Our knowledge of valency allows us to use precise amounts of reactants to produce precise amounts of products. This kind of accuracy is essential in industries like chemical manufacturing, where precision is paramount.
As a leading chemical supplier, being able to predict exactly how much reactant to use also means that at ReAgent we can avoid waste and keep costs down for our customers.
Summary
Valence measures an element’s combining power when it forms molecules or chemical compounds. The concept was first proposed by the British chemist Sir Edward Frankland in 1852. A more accurate model was later developed based on the locations and number of electrons in the outermost shells of atoms. Through our understanding of atomic valency, we can now predict chemical reactions, bonds, molecular structures and chemical properties.
