We’re all familiar with how electrons in an atom are organised into orbitals or shells. Valence electrons are the electrons that exist in the atom’s outermost orbit. According to the Bohr-bury theory, the outermost shell may hold a maximum of 8 electrons. When the outermost shell is entirely filled, there is very little chemical activity. Its combining capacity is likewise reduced to zero.
Nitrogen, for instance, forms a multitude of compounds with hydrogen, including NH3, N2H4, and N3H, in which the nitrogen atoms have valencies of 3, 2, and 1/3, respectively. As a result, the idea of valency as a single number remained unclear. As a result, valency was eventually defined as the number of chemical bonds established by an atom in a molecule.
The combining capacity of an atom in an element with the atom of the same or another element is known as valency. Valency generally represents the outermost electrons in the valency shell.
The outermost shell of an atom is known as a valence shell whereas electrons present in the outermost shell or valence shell are known as valence electrons. Since, valency is defined as the combining the capacity of an atom, combining capacity is determined based on the number of electrons gained, lost, shared by an atom.
Electrons always try to obtain stable electronic configuration by obtaining 8 electrons in the valence shell, so they constantly try to form bonds with the atoms of the same or different elements. Valency is determined based on the number of electrons involved in the formation of a bond.
Let’s study the valency of the first 10 elements of the periodic table as:
Element and its Symbol
Few elements show variable valency because sometimes these elements when combined with other elements under certain different conditions valency of the same element changes.
What is Variable Valency?
Elements showing different valencies in different conditions when reacted with different elements this property is known as variable valency. This property is shown in those elements in which electrons are lost from the penultimate shell to make the atom stable. Mostly d block elements tend to show variable valency.
Examples of variable valency are,
- Iron shows variable valency as 2 and 3.
- Manganese shows variable valency as 2, 4, and 7.
- Copper shows variable valency as 1 and 2.
How to Determine the Valency of an Element?
We can determine the valency of an element by simply knowing its location in the periodic table since they are arranged according to atomic numbers. In order to determine the valency of an element there are different steps involved in it they are:
- The first step to find the valency of an element is to write the respective element electronic configuration and determine the number of electrons in each shell especially in the last shell.
- After determining the number of electrons in the last shell, if the number of electrons is less than or equal to four then the valency of that particular element is going to be equal to the number of electrons in the last shell because valency is defined as combining capacity of an element in order to become stable that particular element will lose these electrons thus having valency equal to a number of valency shell electrons.
- If the valency electrons are more than 4 then the valency of electrons will be equal to the difference between 8 and the number of electrons in the valency shell because to become stable the atom tries to get an octet configuration so valency will be equal to 8 – (number of electrons in the valency shell).
Let’s understand the above concept by considering an example as given below:
Example 1: Determine the valency of Boron.
The electronic configuration of Boron is: 1s2 2s2 2p1
Number of electrons in valency shell = 3
As a number of outermost shell electrons < 4.
This implies the valency of Boron = number of electrons in the outermost shell
Hence, the valency of Boron = 3.
Types of Valency
Based on the bond formed using valence electrons between two atoms valency is divided into two types: Electrovalency and Covalency. Lets discuss them in-depth as:
The number of electrons lost or gained by atoms of an element in order to form an ionic or electrovalent bond so that atoms achieve the nearest stable configuration or inert gas configuration is known as Electrovalency.
Valence is defined as the transfer of electrons from one element’s atoms to another’s atoms during the creation of an ionic connection between the atoms. The number of electric charges lost or acquired by an atom during such a transfer may also be expressed in this manner. The elements will have negative electrovalency if they gain electrons else positive electrovalency if they lose electrons.
Let’s consider the following examples:
- Sodium Chloride (NaCl): Sodium and chlorine atoms form an ionic bond by Sodium losing one electron to get the nearest inert gas configuration that is, to get a Neon gas configuration and chlorine gains one electron and becomes stable. Here, since sodium is losing one electron its electrovalency is 1 and as chlorine is gaining one electron its electrovalency is 1. Na will have positive electrovalency and cl will have negative electrovalency.
- Magnesium Chloride (MgCl2): Magnesium and chlorine atoms form an ionic bond by Magnesium losing two electrons to get the nearest electronic configuration that is, to get the Neon gas configuration and each chlorine atoms gain one electron and become stable. Here, since Magnesium is losing two electrons it will have a positive electrovalency of 2 and chlorine is gaining one electron it will give a negative electrovalency of 1.
The number of electrons shared by two atoms to form a covalent bond so that atoms achieve the nearest stable electronic configuration is known as covalency.
The sharing of electrons in a chemical substance defines valence. It is the number of electron pairs that an atom may share. The covalent bond formation is related to covalency. If atoms share 2 electrons then the covalency of those atoms involved to form a bond is 2.
Let’s consider the following examples:
- Methane (CH4): The number of electrons shared between carbon and hydrogen is 4 so, the covalency of carbon in methane is 4. The Covalency of hydrogen in methane is 1 for each hydrogen atom.
- Nitrogen molecule (N2): The number of electrons shared between two nitrogen atoms here is 3 so. covalency of nitrogen atom is 3.
Differences between Electrovalency and Covalency
Let’s now understand the major differences between electrovalency and covalency as:
|Electrovalency is defined as the number of electrons lost or gained by an atom to form an ionic bond.
|Covalency is defined as the number of electrons shared by atoms to form a covalent bond.
|An ionic bond formation can be explained using Electrovalency
|A covalent bond formation can be explained using Covalency
|The electrovalency explains the formation of ionic bonds.
|However, the covalency of any element leads to the formation of a covalent bond.
|Ionic or Electrovalent bonds will have electrovalency
|Covalent bonds will have covalency
|e.g.: NaCl has electrovalency 1.
|e.g.: Nitrogen molecule has covalency 3.
Problem 1: Calculate maximum covalency that can be shown by Aluminium?
The maximum covalency that can be shown by Aluminium is 6. As Aluminium has vacant d orbitals so the electrons can pair up to 5 electrons here and s and p orbitals have 2 and 1 electrons in them. So, the total sum is 8 as Aluminium cannot have 8 electrons due to its small size its covalency is 6.
Problem 2: Explain the causes of variable valency?
If electrons from the penultimate shells are lost then variable valency is shown in elements. The two main causes of variable valency are differences in energy levels of orbitals and the inert pair effect.
Problem 3: What is the sign of valency?
Valency doesn’t show any sign it can neither be positive or negative. By losing or gaining electrons elements will get charge which will have a sign.
Problem 4: Are Valency and Oxidation numbers both are same?
No, valency and oxidation numbers both are different. Valency is defined as combining capacity of an element whereas Oxidation number is defined as the number of electrons lost or gained by an atom while forming a bond. Valency cannot be positive or negative but oxidation numbers can be positive or negative.
Problem 5: Calculate electrovalency of Aluminium in AlCl3?
The electrovalency of aluminium in Alcl3 is +3. Here, Aluminium loses 3 electrons and 3 chlorine atoms take an electron each and form an electrovalent bond. Since Aluminium is losing 3 electrons and electrovalency is calculated based on the number of electrons lost or gained by an atom electrovalency of aluminium is +3.
Problem 6: Does s-block elements show variable valency?
The s-block elements don’t show variable valency because these elements lose one electron and get stable inert gas configuration and the second ionization potential is very high. Since these elements are stable after losing one electron they don’t show variable valency.
Problem 7: Which elements have zero valencies?
Noble gases have zero valencies because these elements have stable configuration and as valency is defined as combining capacity of an element and Noble gases already have a complete outer shell filled they don’t react with other atoms to become stable.
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