Valence of chemical elements

Definition of valence

Valence is defined as the number of bonds that an atom of a chemical element can create by linking with other atoms.

What else should we know about the valence of chemical elements?

• The valence of elements is expressed in Roman numerals;
• in the case of chemical elements that combine into compounds by creating covalent bonds, valence can be determined based on a structural formula (the valence of an element is equal to the number of bonds created by a given atom of that element in the compound);
• the valence of elements in the elemental state is always zero;
• for ions, the valence of an element is equal in number to the charge of that ion (excluding positive and negative signs).

Valence and oxidation state of a chemical element

The terms of valence and oxidation state of an element are often used interchangeably. Is this justified? Unfortunately, such an approach to these concepts is incorrect. The main reason why they are mistaken is their graphic representation: both terms use Roman numerals.

The oxidation state of an element that composes a specific substance is defined as the number of positive or negative charges which could be attributed to the atoms of that element if the molecules of that substance had an ionic structure, i.e., if they were capable of decomposing into ions. What is important, the term ‘oxidation state’ is conventional, as by definition it assumes the presence of ionic bonds only, which is not always the case.

Therefore, while the oxidation state determines the charge of a hypothetical ion produced by decomposition of a chemical compound, the valence defines the number of bonds that the element can form. In addition, the oxidation state takes positive and negative values, unlike the valence, which is always positive.

Does each element has only one valence?

Different chemical elements do not interact in the same way. In consequence, their valence varies based on the element they form a bond with. We always have to state the element’s valence in the compound if it takes more than one value.

Many chemical elements feature variable valence. Which chemical compound is formed by the element, and which are the other components, determines the element’s valence.

For instance, one of such elements is nitrogen. Its maximum valence is V. It may also take lower values. For example, in trioxonitric(V) acid, the valence of nitrogen is V, while in dioxonitric(III) acid its valence is lower and equals III.

There are more such examples. An aid in determining the valence of a chemical element is provided by the periodic table of elements. To give an example, the valence of group I elements is I, and the valence of group II elements is II. Chlorine and other group 17 metals, which come last in the formula (e.g., …Cl), have a valence of I. The periodic table also makes it possible to determine the maximum valence of elements from the main groups in chemical compounds with oxygen and hydrogen.

Determining the formulas of chemical compounds using their valence

In nature, chemical elements can be more or less prone to interaction to form chemical compounds. With the use of the globally recognised chemical letter symbols and the valence of individual elements, we note down chemical compounds using formulas. We distinguish structural, semi-structural and molecular formulas.

Structural formula

With this formula, we can show the structure of a molecule of a specific chemical compound. It includes the type and quantity of atoms as well as all bonds existing between them.

Semi-structural formula

In this formula, we create a sort of grouping of elements: we group carbon with hydrogens separately from functional groups. Semi-structural formulas, in their notation, show the bonds existing between subsequent atoms of carbon and functional groups.

Molecular formula

The most common formula used for symbolic description of a chemical compound, for example, the molecular formula of sodium chloride (common salt), is NaCl. It includes the type and quantity of atoms.

Thus, when we know a few fundamental rules concerning the valence of chemical elements, we can easily note down the molecular formula and structural formula of a molecule consisting of two chemical elements:

• the first step is to write down the chemical symbols of the elements forming the compound next to each other;
• next, in the upper right corner, with Roman numerals, we write their valences, which we then write under the elements below (crosswise!);
• the noted valences form the mass ratio of the elements included in the compound. If this is not the lowest ratio, the numbers should be divided by their common dividend;
• then the numbers should be written (in Arabic numerals) in the lower right corner of the elements’ chemical symbols (do not write number one).

The structural formula of a chemical compound is created in a similar way:

• first write down the letter symbols of the elements forming the compound (such as the number of atoms of the element that was determined earlier, based on the molecular formula);
• by each symbol, write down as many dots as is the valence of the corresponding element;
• link the dots written between the atoms (no dot may be left); each link stands for a chemical bond.

Valence of chemical elements determined ‘with the naked eye

Is it possible to define the valence of a chemical element in a chemical compound ‘with the naked eye’? It turns out to be possible, but we should remember to be a little wary when we use that method.

Many of us associate the world of chemistry with flasks containing solutions with a variety of intense colours. Such solutions can be obtained thanks mainly to metals that are shown in block d of the periodic table of elements. Most of these metals have intense colours which, with a certain degree of probability, can indicate the element’s valence. To give an example:

• iron(II) salts in solutions are pale green and iron(III) salts are yellow,
• cobalt(II) salts are pink and cobalt (III) salts are blue,
• chromium(II) salts in solutions are blue and chromium(III) salts have a violet colour.