Valency: Definition, Calculation, Examples, Valence Electrons

The chemical formula for water is \({{\text{H}}_2}{\text{O,}}\) as we all know. How many times have we wondered why “\({{\rm{H}}_{\rm{2}}}{\rm{O}}\)” is used instead of something else? Why does H and O atoms combine only in this way to form water? In chemistry, valency of an element is defined as the measure of its combining capacity with other atoms when it forms chemical compounds. The valency of an element is related to how many electrons are present in the outer shell of the atom of that element. Therefore, elements in the same group of the periodic table have the same valency. This article discusses the concept of valency in detail. Read on to learn more.

What is Valency?

The number of valence electrons of an element that actually takes part in chemical reactions is called the valency of that element. The combining capacity, (affinity) of an atom is determined by the number of hydrogen atoms that it can combine with. For instance, in hydrogen chloride, chlorine has a valence of 1; in water, oxygen has a valence of 2; in ammonia, nitrogen has a valence of 3 and in methane, carbon has a valence of 4. Chlorine, as it has a valence of one, can be substituted for hydrogen. Similarly, in phosphorus pentachloride, – PCl5, phosphorus has a valence of 5. Valence diagrams of a compound are drawn to represent the connectivity of the elements, with lines drawn between two elements to indicate bonds.

Definition of the Valency of an Atom

The valency of an atom is its combining capacity in chemical reactions. It is defined as follows:

The valency of an atom is equal to the number of valence electrons that this atom can gain or lose during chemical reactions.

Or

The number of hydrogen atoms or chlorine atoms or double the number of oxygen atoms with which one atom of an element combines is called its valency.

Example of Valency of An Atom

Note: An atom loses or gains electrons to form ionic bonds and attain a stable electronic configuration similar to its nearest noble gas by completing its octet.

The valency of an atom is equal to the number of valence electrons which this atom contributes for sharing with other atoms during chemical reactions.

Note: An atom shares its electrons to form a covalent bond and attains a stable electronic configuration similar to its nearest noble gas by thus completing its octet.

Principle of Calculation of Valency

The number of electrons in the valence shell of an atom that actually takes part in forming a chemical bond is equal to its valency.

Calculation of the Valency of a Metal

An element is called a metal if it has one or two or three electrons in its valence shell (except hydrogen and helium, which are non-metals). Metal tends to lose its valence electrons in chemical reactions and complete its octet. Therefore,

valency of metallic element \( = \) number of electrons in its valence shell

Valency of Aluminium

Aluminium’s atomic number is \(13,\) which means its electrical configuration is \(2,8,3.\) Its valency is \(3\) because it contains \(3\) electrons in its valence shell and must lose \(3\) electrons to complete its octet.

Calculation of the Valency of a Non-metal

An element is called a non-metal if it has \(5\) or \(6\) or \(7\) or \(8\) electrons in its valence shell.

Therefore, a non-metal needs \(3\) or \(2\) or \(1\) or \(0\) electrons to complete its octet. Thus valency of a non-metallic element \( = 8 – \) number of electrons in its outermost shell.

Thus, it is clear from the electronic configuration of some non-metals

Example 1: The valency of sodium is one and not \(7.\) Give reason?
Answer: Electronic configuration of sodium \(\left({{\text{Na}}} \right)\)
\({\text{Na}}\) (Atomic number \( = 11\)) \( = 2,8,1\)
(i) Sodium has one electron more than its nearest noble gas neon \(\left({2,8} \right).\) Therefore, sodium can lose one electron from its valence shell to complete its octet. Hence the valency of sodium is one.
(ii) The valency of sodium maybe \(7\) only when it gains \(7\) electrons in its valence shell. But gaining \(7\) electrons is comparatively difficult than losing a single electron. Therefore, the valency of sodium is not \(7.\)

Example 2: The valency of chlorine is \(1\) and not \(7.\) Give reason?
Answer: Electronic configuration of chlorine
\({\text{Cl}}\) (atomic number \( = 17\)) \( = 2,8,7\)
(i) Chlorine needs one electron to complete its octet in the valency shell. Therefore, the valency of chlorine is one.
(ii) The valency of chlorine maybe \(7\) only when it loses \(7\) electrons from its valence shell. But losing \(7\) electrons is much more difficult than gaining a single electron. Therefore, the valency of chlorine is not \(7.\)

Valency of Carbon

The valency of carbon is four. The outer electronic configuration of carbon is \({\text{1}}{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^2}.\) As a result, it has four valence electrons. It forms four covalent bonds by sharing its four valence electrons.

Valency of Nitrogen

The outermost shell of the nitrogen atom possesses \(5\) electrons; it may accept \(3\) electrons to complete the octet structure. As a result, nitrogen has a valency of \(3.\)

Valency of Oxygen

The valency of oxygen is two. The outer electronic configuration of oxygen is \({\text{1}}{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^4}.\) As a result, it has six valence electrons. According to the formula, the valency of a non-metallic element \( = 8 – \) number of electrons in its outermost shell.

\( = 8 – 6 = 2.\)

Variable Valency

There are some elements that show different valences in a different compound.

If the element shows more than one valency in its compounds, the element is said to exhibit variable valency.

Example: The valency of \({\text{Fe}}\) in \({\text{FeC}}{{\text{l}}_2}\) is \(2\) and that of \({\text{FeC}}{{\text{l}}_3}\) is \(3.\)
In \({\text{C}}{{\text{u}}_2}{\text{C}}{{\text{l}}_2}\) and \({\text{CuC}}{{\text{l}}_2}\) the valency of \({\text{Cu}}\) is \(1\) and \(2,\) respectively.
In \({\text{SnC}}{{\text{l}}_2}\) and \({\text{SnC}}{{\text{l}}_4}\) the valency of \({\text{Sn}}\) is \(2\) and \(4,\) respectively.

How to Name the Element with two Different Valences?

The compound in which the element shows the higher valences is indicated by the suffix -ic. The compound in which the element shows the lower valences is indicated by the suffix -ous.

1. For example, iron exhibits two valences \(2\) and \(3\)
2. Iron in its lower valency \(2\) is named ferrous.
3. Iron in its higher valency \(3\) is called ferric.

Note: The symbol of elements with variable valency remains the same, even though the valency changes.

Sometimes the variable valency is represented by Roman letters.

Example:
Ferrous is written as \({\text{Fe}}\,\left({{\text{II}}} \right).\)
Ferric is written as \({\text{Fe}}\,\left({{\text{III}}} \right).\)

Variable Valency of Non-metals

Example 1: Valence shell electronic configuration of phosphorus in the ground state is

Here the valency of phosphorus is \(3\) due to the presence of \(3\) unpaired electrons. The electronic configuration of phosphorus in the first excited state is given above.

Here one electron has excited from \(3{\text{s}}\) orbital to \(3{\text{d}}\) orbital, so the number of unpaired electrons is \(5.\) So, it also exhibits the valency of \(5.\) Hence the valences of phosphorus are \(3\) and \(5.\)

Example 2: Valence shell electronic configuration of sulphur in the ground state is

Here the valency of sulphur is \(2\) due to the presence of two unpaired electrons. The electronic configuration of sulphur in the first excited state is

Here one electron has excited from \(3{\text{p}}\) orbital to \(3{\text{d}}\) orbital, so the number of unpaired electrons is \(4.\) Hence the valency of sulphur is \(4.\)

Here one electron has excited from \(3{\text{s}}\) orbital to \(3{\text{d}}\) orbital, and so the number of unpaired electrons is \(6.\) Hence the valency of sulphur is \(6.\) Hence the valences of sulphur are \(2,4\) and \(6.\)

Relation Between Valence Electrons and Chemical Properties of Elements

In general, the chemical properties of the elements are linked with the electrons present in the valence shells of their atoms or with their valency.

1. Elements with the same number of valence electrons or the same valency have similar chemical properties. For example, lithium \(\left({{\text{Z}} = 3} \right),\) sodium \(\left({{\text{Z}} = 11} \right)\) and potassium \(\left({{\text{Z}} = 19} \right)\) have all one valence electrons and show valency equal to \(1\) in their compounds.
2. Elements with either two or eight valence electrons in outermost or valence shells have zero valences. This means that their atoms neither lose nor gain electrons and are very little chemical reactive. These are known as noble gas elements. For example, helium \(\left({{\text{Z}} = 2} \right),\) neon \(\left({{\text{Z}} = 10} \right)\) and argon \(\left({{\text{Z}} = 18} \right)\) have zero valency.
3. Elements are having \(1,2,\) and \(3\) valence electrons tend to lose these electrons. These are generally metals.
4. Elements are having \(5,6,\) and \(7\) valence electrons tend to gain \(3,2,1\) electron, respectively. These are known as non-metals.
5. Elements that have \(4\) valence electrons have the characteristics of both metals and non-metals. These are called semi-metals or metalloids. Both carbon and silicon have four valence electrons. Carbon is non-metal, but silicon is a semi-metal or metalloid.

Noble gas elements (\({\text{He}},{\text{Ne}},{\text{Ar}},{\text{Kr}},\) and \({\text{Rn}}\)) are chemically inert since their valence shells are completely filled. That is, their atoms have \(8\) electrons in the valence shell (Helium has two valence electrons). They have zero valences and, therefore, no combining capacity.

Summary

The combining power or capacity of an element is known as its valency. It can be used to find out how the atoms of an element will combine with the atoms of another element to form a chemical compound.

The atoms having one, two, three, or four valence electrons normally lose these to the combining atoms, and they show valences of one, two, three, or four, respectively. The atoms having \(5,6,\) or \(7\) valence electrons generally take up \(3,2,\) or \(1\) electron respectively from the atoms of the other elements and show the valences of \(3,2,\) or \(1,\) respectively. This article learned about the definition of valency, calculation of valency of a metal and non-metal, variable valency, and valence electrons.

FAQs on Valency

The answers to the frequently asked questions on valency are given below:

Chemistry Related Articles,

We hope this article on ‘Valency’ has helped you. If you have any queries, drop a comment below and we will get back to you.