VALENCY AND VALANCE ELECTRONS

We have studied earlier about the arrangement of electrons in different shells/orbits. “The electrons present in the outermost shell/orbit of the atom of an element are called valence electrons.” In all chemical reactions, only the electrons present in outermost orbit will take part in the reaction. As these electrons are present farthest from the nucleus, hence the force of attraction on these electrons by the nucleus is minimum. According to Bohr-Bury scheme a maximum of 8 electrons can be accommodated in the outermost orbit of an atom. Helium and hydrogen are exception as they can have only two electrons is their outermost shell.

What does the term Valency mean?

Valency is the combining capability of an element. It is always an integer. It has no plus or minus sign. A electrons which present in the outer shell of an atom are known as "valence electrons." We can say that valence is the number of electrons that an element can lose or gain to achieve stability. Generally, valency is used to write the chemical formula of a compound. This means that we can use valency to know how an element's atoms will bond with another element's atoms.

Concept

In atoms, the electrons are arranged to revolve around the nucleus in different orbitals (shells). These orbitals are represented as K, L, M, N, and so on. The atom of valence electron remains in the outermost electron shell. These electrons in the outer shell participate in any chemical reaction because they generally contain more energy than electrons that are present in different orbits.

According to the octet rule, the outermost orbit of an atom will have a maximum of 8 electrons to become stable. However, if the outer orbital is filled, then sometimes very little to no chemical activity is observed in the element.

Valency of an Atom

The concept of valency arises from the study of inert elements. Inert elements are also called noble gases. They have 8 valence electrons (octet) in their outermost orbit/shell or valence shell except helium which has 2 electrons (duplet). Apart from these elements, all other elements have less than 8 electrons in their valence shell. To attain stability, these atoms lose, gain or share electrons with other atoms to complete their octet. Thus, valency of the atom of an element can be defined as follows:

“The combining capacity of the atoms of an element is known as valency.”

Or

“The number of electrons gained, lost or shared by atom of an element in order to complete its octet (or duplet) or to attain stable configuration is known as the valency of the element”

Calculation of Valency

To calculate the valency of an element, the electronic configuration of the element must be written first and then the valency is calculated. The valency of an element can be calculated as follows:

• Elements having 1, 2, 3 and 4 electrons respectively in their valence shell: For these elements valency is equal to the number of electrons present in their valence shell.
• Elements having more than 4 electrons in their valency shell: For these elements having more than 4 electrons in their valence shell, valency can be calculated as follows:

Valency = 8 – Number of valence electrons

Valency of first 30 elements

Below is the Valency table of first 30 elements

A valency or valency chart(table) helps determine how many atoms of an element combine with another element to form any chemical formula. Another important use of the valence of elements is to find or derive the formulas of compounds. If we know the valence of the elements, we can quickly write the formulas of the compounds of these elements. For example -

• The formula of the compound carbon tetrachloride:

The symbol for carbon is C, and chlorine is Cl. Their valences are 4 and 1. The formula of the tetrachloride compound will be CCl ₄ as: chlorine loses one electron, and carbon gains four electrons to complete its octet.

• The formula of magnesium chloride compound

The symbol for magnesium chloride is Mg and for chlorine is Cl. Their valances are 2 and 1. The formula of magnesium chloride is MgCl ₂ because magnesium loses two electrons, and chlorine gains one electron to form the compound.

Frequently Asked Question (FAQs)

Q1. Why do we need valency?

Ans. We need the valency of an element because it determines how strong the bond between atoms will be. The higher the valencY, the stronger the bond. That is why high-strength elements are often used in chemical reactions - they form strong bonds with other atoms.

Q2. How many types of valency are there?

Ans. There are three types of valency, i.e., primary or classical, coordinate, and nonclassical.

Q3. Can valency be negative?

Ans. Loss of electron or electron gain called atom charge, Positive Charge will be attained by donating an electron and negative charge vice versa. So valence has no sign, and Charge has both positive and negative signs.