Variable Valency: Determination, Causes and Electrovalency

The combining power of elements or atoms is called valency. There are two types of Valency: Electro valency or Valency in Ionic Compounds and covalency or Valency is Covalent Compounds, but when an atom or an element shows various types of valency that are known as Variable Valency.  

Keyterms: Valency, Electro valency, Ionic Compounds, covalency, Covalent Bind, atom, element, Compounds

Read More: Classification of Elements and Periodicity in Properties

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Variable Valency 

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Variable valency is the power of elements to represent different valencies in different compounds. As the name suggests, Variable valency doesn’t have a fixed pattern. To get the Variable valency of any element, its electronic configuration is necessary. When a substance such as copper (Cu) shows multiple vacancies, we know it as variable valency. 

• Valency can be referred to as the number of electrons which are donated or accepted by any element.
• The capacity of an element to release or accept electrons to form a stable electronic order or combination will depend on its valency. 
• Variable Valency occurs because of different conditions during a chemical reaction. The same element goes through different situations; Consequently, the Valency of that element changes accordingly. 

Variable Valency Example

Copper’s atomic number is 29. So, Copper’s Electric Configuration will be 2, 8, 18 (Penultimate shell which means the second last orbit), 1 (Valence).  

• Here, the second orbit and the valence have one electron. 
• The electron will travel from one shell (second last shell) to another (the last shell) to become stable or two to make two electrons in the last shell. 
• This electronic configuration shows different valencies in the outer shell of the Penultimate shell and the last shell of the last orbit.
• The valency of copper in the penultimate shell is 1 while the valency of copper in the last shell is 2 because one electron from the Penultimate Shell has jumped to the last orbit. 
• Sometimes icons also show variable valency. For example: Fe+ 2 oxides. 

Check out Important Question on: Valency to Copper and Iron

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Cause of Variable Valency 

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Some elements represent Variable Valency by reason of different electronic configurations. It happens due to the loss of electrons from one shell to another. 

1) The reason for losing extra electrons is a matter of specific electrons. 

2) The process of Jumping electrons from 1 shell to another is due to the presence of vacant orbits in the same sub-shells. 

Examples:  

1. Sulfur’s atomic number is 16. In its 3^rd orbit, there is a presence of vacant elements in the d-block, due to which 6 electrons in the third orbit don’t combine with others, and occupy these orbits. That’s the reason sulfur shows unfixed valencies. 
2. The atomic number of Iron is 26. So, its electronic configuration, in the last three shells, will be 6 (third last shell), 2 (Penultimate Shell), 6 (last shell). This doesn’t make a stable connection.

• Here, electrons will jump from one shell to another until they get a stable connection or they become stable. 
• The capacity to form a bond with other elements depends on vacant blocks in the orbit and the configuration. 
• If the configuration is not strong enough, these electrons will transfer.
• Once an electron becomes stable, they show its valency. And it is that the valency would be different if two processes take place: 

1. Pairing of two electrons.
2. Jumping of electrons from one shell to another.

So, the reason for showing different valency is clear. Elements do this kind of activity to acquire stability. Sub-shells with more space show more exchangeable energy which lowers the energy of the compound. And makes more stable compounds. That is the inert pair effect. 

Group of Elements that show Variable Valency

There are many elements with Variable Valencies which includes:

• Copper (Cuprous, Cupric oxides) 
• Iron
• Mercury
• Silver 

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Valence in Variable Valency 

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The combining power of elements or atoms is called valence. In simple terms, Valence is the maximum number of electrons that show the ability of an atom to combine with another.

• Electrons are called the valence electrons which are present in the outermost shell.

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Variable Electrovalency in Variable Valency 

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When the number of electrons gained or lost by an atom to make an ionic bond (bond with the opposite charges such as metallic to non-metallic) show more than one valency, this transition is known as Variable Electrovalency. 

• For example, tin has an electrovalency of 2 & 4.

Read More: Periodic Properties of Elements

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Inert Pair Effect in Variable Valency 

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The outermost electrons in an atom remain unchanged during the formation of compounds. It simply means that the outermost electron shell doesn’t pair with the ionized compounds.

• This effect occurs in heavier chemical elements such as elements of groups 13 to 16.
• Only elements of the p-block show this effect. 

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Energy differences between Orbitals in Variable Valency 

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The orbit of an electron will be decided by its energy level. And the orbit energy depends on the principle of the Quantum Number (n) & Azimuthal Quantum Number (1).

• As per the principle, s orbital electrons will have more negative or lesser amounts of energy as compared to p orbital electrons.
• p orbital electrons will have a lesser amount of energy than that of d orbital electrons.
• When the atomic number increases, the orbital energy decreases in the same subshell.

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Determination of Valencies in Variable Valency 

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We determine valency by the outermost shell's electrons. Let’s understand it with an example; 

• Chlorine has 7 electrons in the outermost shell, then the valency of chlorine will be 8 - number of electrons in the outermost shell of chlorine (Outermost shell can’t have more than 8 electrons. That’s why 8 - number of electrons in the outermost shell of chlorine).

Valency of Chlorine = 8 - outermost shell’s electrons 

= 8 - 7 

= 1

Read Also: Chemical Bonding and Molecular Structure

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Things to Remember 

• Variable valency is a part of CBSE class 11 syllabus of unit 3 Classification of elements and Periodicity in Properties.
• It consists of a total of 6 periods and 4 marks. 
• Variable valency is shown by transition elements. 
• The elements present in s-block (sodium, calcium, magnesium) don’t show variable valency. 
• The d-block elements are half-filled when they show variable valency. When the orbitals of such elements are full, they don’t show variable valency. For example, Cadmium doesn’t show variable Valency. 
• In the Halogen group, 1 is the valency of the elements. When the K shell is completely filled, the valency will be zero. 
• Variable valency increases with the atomic numbers. 
• d-shell has more stability for elements. 
• Hund's rule states that before filling with the antiparallel spins electrons primarily fill up shells with parallel spins. 
• Chemist. G.N. Lewis invented Valency. 
• Shell 1 valence electrons has subshell s, Shell 2 has two subshells; s and p, and Shell 3 has s, p, and d.

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