In the world of chemistry, we learn from our observations and try to theorise different observations well supported by logic. Hybridisation is one such concept that we learned by observing the bonding of different atoms with each other. In this article, we will discuss in detail about Hybridisation, its types, rules, etc.

Hybridisation is the process of mixing atomic orbitals of different shapes with approximately the same energy to produce the same number of hybrid orbitals with the same shape, energy, and orientation, with the least amount of repulsion between them. Continue reading to know more.

Purpose of Hybridisation

Hybridisation is a concept which deals with the intermixing of orbitals of the same shell with each other to form a new hybrid orbital that is used in bond formation by an atom.

To understand it in more depth, let us take an example of a carbon atom having an atomic number of six, which means that an atom of carbon consists of six protons, and as an atom is neutral, it consists of six electrons as well.

The electronic configuration of a carbon atom thus can be written as \({\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1.\) To make covalent bonds, one atom must have a half-filled orbital. Hence, accordingly, the carbon atom should only make two covalent bonds with \({\rm{2p}}_{\rm{x}}^1\,{\rm{2p}}_{\rm{y}}^1\) (Half-filled orbital), but the number of covalent bonds a carbon atom can make is well known by us, which is four.

Now the question comes out is how is that happening? And as per the electroscope, it is well established that a carbon atom makes four covalent bonds and has a covalency of four in a compound. Another theory of hybridisation gave the solution for this.

What is Hybridisation?

To overcome the above problem, chemists suggested that one pair of an electron in a carbon atom in an orbital becomes unpaired by jumping into another empty orbital of the same shell without consuming excessive energy.

As in the case of the carbon atom, the number of electrons present unpaired after excitation is;. \({\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_x^1{\rm{2p}}_y^1{\rm{2p}}_{\rm{z}}^1\) that is four Hence the number of half-filled carbon orbitals is equal to four, contributing to the fact that carbon has four covalencies and explains the tetravalent nature of the carbon atom.

\({{\rm{C}}_6}{\rm{ = 1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^{\rm{1}}\) (Ground state)

\({{\rm{C}}_6}{\rm{ = 1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1{\rm{2p}}_{\rm{z}}^1\) (Excited state)

Also, with this, the covalent bond formed with overlapping of \({\rm{s}}\) orbital should have less energy as compared to that of \({\rm{p}}\) orbitals. Still, experimental data shows no significant difference in their energy levels.

The bonds with three \({\rm{p}}\) orbitals should be perpendicular to each other as they are aligned with the three axes of the coordinate system, and the bond formed with s orbital may have any direction, but in reality, this is not the case. To overcome this issue, the concept of hybridisation was introduced.

Hybridisation is defined as the mixing of the atomic orbitals belonging to the same atom but having slightly different energies, so that redistribution of energy takes place between them, resulting in the formation of new orbitals of equal energies and identical shapes. The new orbitals thus formed are known as hybrid orbitals.

To understand it in simple terms or with an analogy, when a can of blue paint is mixed with a can of white paint, it gives a different colour of sky blue, which is neither totally blue or neither pure white and is somewhere between both; also two cans of white and blue paint give two cans of sky blue paint as well.

Important Points on Hybridisation

1. Orbitals of similar energies(same shell) and of the same atom can undergo hybridisation.
2. The number of hybrid orbitals produced is equal to the number of orbitals mixed.
3. Half filled and completely filled orbitals, both participate in the hybridisation.
4. Hybridisation occurs during the time of bonding only.
5. Type of hybridisation indicates the geometry of the molecule.
6. Hybrid orbitals are equivalent in energy and shape.
7. Hybrid orbitals form only sigma bonds.

Types of Hybridisation

1. Diagonal or sp Hybridisation

When one s and one \({\rm{p}}\) orbitals belonging to the same main shell of an atom mix together to form two new equivalent hybrid orbitals, this type of hybridisation is known as \({\rm{sp}}\) hybridisation or diagonal hybridisation.

The shape of the molecule formed after this hybridisation is linear, having an angle of \({\rm{18}}{{\rm{0}}^{\rm{^\circ }}}{\rm{.}}\) The hybrid orbitals formed have \({\rm{50\% }}\,{\rm{s}}\) character and \({\rm{50\% }}\,{\rm{p}}\) character.

For example, while the bond formation of \({\rm{Be}}{{\rm{H}}_2},\)

\({\rm{Be}} = 1\;{{\rm{s}}^2}2\;{{\rm{s}}^2}2{{\rm{p}}^0}\) (Ground state)

\({\rm{Be}}\,{\rm{ = }}\,{\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{{\rm{1}}\:}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^0{\rm{2p}}_{\rm{z}}^1\) (Excited state)

So when two hydrogen atoms arrive for bond formation, they make a bond with two sp hybrid orbitals made after intermixing of \({\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_{\rm{x}}^1\) orbitals and have a linear shape.

2. Trigonal or \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) Hybridisation

When one \({\rm{s}}\) and two \({\rm{p}}\) orbitals belonging to the same main shell of an atom mix together to form three new equivalent hybrid orbitals, this type of hybridisation is known as \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridisation or trigonal hybridisation.

The shape of the molecule formed after this hybridisation is a triangular planner having an angle of \({\rm{12}}{{\rm{0}}^{\rm{^\circ }}}{\rm{.}}\) The hybrid orbitals formed have approximately \({\rm{33}}{\rm{.33\% }}\,{\rm{s}}\) character and \({\rm{66}}{\rm{.66\% }}\,{\rm{p}}\) character.

For example, while the bond formation of \({\rm{B}}{{\rm{H}}_{\rm{3}}}{\rm{,}}\)

\({\rm{B}}\,{\rm{ = }}\,{\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^0{\rm{2p}}_{\rm{z}}^0\) (Ground State)

\({\rm{B}}\,{\rm{ = }}\,{\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1{\rm{2p}}_{\rm{z}}^0\) (Excited state)

So when three hydrogen atoms arrive for bond formation, they make a bond with three \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybrid orbitals made after intermixing of \({\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1\) orbitals and have a triangular planar shape.

3. Tetrahedral or \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) Hybridisation

When one s and three \({\rm{p}}\) orbitals belonging to the same main shell of an atom mix together to form four new equivalent hybrid orbitals, this type of hybridisation is known as \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridisation or tetrahedral hybridisation.

The shape of the molecule formed after this hybridisation is tetrahedral, having an angle of \({\rm{10}}{{\rm{9}}^{\rm{o}}}\,{\rm{28′}}{\rm{.}}\) The hybrid orbitals formed have approximately \({\rm{25\% }}\,{\rm{s}}\) character and \({\rm{75\% }}\,{\rm{p}}\) character.

For example, while the bond formation of \({\rm{C}}{{\rm{H}}_{\rm{4}}}{\rm{,}}\)

\({\rm{C = 1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1{\rm{2p}}_{\rm{z}}^0\) (Ground State)

\({\rm{C = 1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1{\rm{2p}}_{\rm{z}}^1\)(Excited State)

So when four hydrogen atoms arrive for bond formation, they make a bond with four \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybrid orbitals made after intermixing of \({\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1{\rm{2p}}_{\rm{z}}^1\) orbitals and have a tetrahedral shape.

4. Trigonal Bipyramidal or \({\rm{s}}{{\rm{p}}^{\rm{3}}}{\rm{d}}\) Hybridisation

When one s, three \({\rm{p}}\) and one \({\rm{d}}\) orbitals belonging to the same main shell of an atom mix together to form five new equivalent hybrid orbitals, this type of hybridisation is known as \({\rm{s}}{{\rm{p}}^{\rm{3}}}{\rm{d}}\) hybridisation or Trigonal Bipyramidal.

The shape of the molecule formed after this hybridisation is Trigonal Bipyramidal having an angle of \({\rm{12}}{{\rm{0}}^{\rm{^\circ }}}\) within the equatorial triangular bonds and \({\rm{9}}{{\rm{0}}^{\rm{^\circ }}}\) with the axial and equatorial bonds.

For example, while the bond formation of \({\rm{P}}{{\rm{F}}_{\rm{5}}}{\rm{,}}\)

\({\rm{P}}\,{\rm{ = }}\,{\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{p}}^{\rm{6}}}{\rm{3}}{{\rm{s}}^{\rm{2}}}{\rm{3p}}_{\rm{x}}^1{\rm{3p}}_{\rm{y}}^1{\rm{3p}}_{\rm{z}}^1{\rm{3}}{{\rm{d}}^{\rm{0}}}\) (Ground State)

\({\rm{P}}\,{\rm{ = }}\,{\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{p}}^{\rm{6}}}{\rm{3}}{{\rm{s}}^{\rm{1}}}{\rm{3p}}_{\rm{x}}^1{\rm{3p}}_{\rm{y}}^1{\rm{3p}}_{\rm{z}}^1{\rm{3}}{{\rm{d}}^{\rm{1}}}\) (Excited State)

So when five fluorine atoms arrive for bond formation they make a bond with four \({\rm{s}}{{\rm{p}}^{\rm{3}}}{\rm{d}}\) hybrid orbitals made after intermixing of \({\rm{3}}{{\rm{s}}^{\rm{1}}}{\rm{3p}}_{\rm{x}}^1{\rm{3p}}_{\rm{y}}^1{\rm{3p}}_{\rm{z}}^1{\rm{3}}{{\rm{d}}^{\rm{1}}}\) orbitals and have a Trigonal Bipyramidal shape.

There are a few more types of hybridisation that we may see in this table, with a few good examples:

Types of Hybridisation	Atomic orbitals involved	Representing directions of hybrid orbitals formed Along with bond angles	Shape of the molecule	Examples
1. \({\rm{sp}}\)	\({\rm{one}}\,{\rm{s}}\,\,{\rm{ + one}}\,{\rm{p}}\)		Linear	\({\rm{BeC}}{{\rm{l}}_2},{\rm{Be}}{{\rm{H}}_2},{{\rm{C}}_2}{{\rm{H}}_2},{\rm{HgC}}{{\rm{l}}_2}\)
2. \({\rm{s}}{{\rm{p}}^{\rm{2}}}\)	\({\rm{one}}\,{\rm{s}}\,\,{\rm{ + two}}\,{\rm{p}}\)		Triangular planar	\({\rm{B}}{{\rm{F}}_3},{\rm{BC}}{{\rm{l}}_3},{{\rm{C}}_2}{{\rm{H}}_4},{\rm{NO}}_3^ – ,{\rm{CO}}_3^{2 – }\)
3. \({\rm{s}}{{\rm{p}}^{\rm{3}}}\)	\({\rm{one}}\,{\rm{s}}\,\,{\rm{ + three}}\,{\rm{p}}\)		Tetrahedral	\({\rm{C}}{{\rm{H}}_4},{\rm{CC}}{{\rm{l}}_4},{\rm{SnC}}{{\rm{l}}_4},{\rm{NH}}_4^ + \)
4. \({\rm{ds}}{{\rm{p}}^{\rm{2}}}\)	\({\rm{one}}\,{\rm{d}}\,\left( {{{\rm{d}}_{{{\rm{x}}^{\rm{2}}}{\rm{ – }}{{\rm{y}}^{\rm{2}}}}}} \right){\rm{ + }}\,{\rm{one}}{\mkern 1mu} {\rm{s}}\,{\rm{ + }}\,{\rm{two}}\,{\rm{p}}\)		Square planar	\({\left[ {{\rm{Ni}}{{({\rm{CN}})}_4}} \right]^{{\rm{2 – }}}},{\left[ {{\rm{PtC}}{{\rm{l}}_4}} \right]^{{\rm{2 – }}}}\)
5. \({\rm{s}}{{\rm{p}}^{\rm{3}}}{\rm{d}}\)	\({\rm{one}}\,{\rm{s + three}}\,{\rm{p + one}}\,{\rm{d}}\left( {{{\rm{d}}_{{{\rm{z}}^2}}}} \right)\)		Trigonal bipyramidal	\({\rm{P}}{{\rm{F}}_5},{\rm{PC}}{{\rm{l}}_5}\)
6. \({\rm{s}}{{\rm{p}}^{\rm{3}}}{{\rm{d}}^{\rm{2}}}\,{\rm{or}}\,{{\rm{d}}^{\rm{2}}}{\rm{s}}{{\rm{p}}^{\rm{3}}}\)	\({\rm{one}}\,{\rm{s + three}}\,{\rm{p + two}}\,{\rm{d}}\left( {{{\rm{d}}_{{{\rm{x}}^2} – {{\rm{y}}^2}}}\;{\rm{and}}\;{{\rm{d}}_{{{\rm{z}}^2}}}} \right)\)		Octahedral	\({\rm{S}}{{\rm{F}}_6},{\left[ {{\rm{Cr}}{{\rm{F}}_6}} \right]^{3 – }},{\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]^{3 + }}\)
7. \({\rm{s}}{{\rm{p}}^{\rm{3}}}{{\rm{d}}^{\rm{3}}}\)	\({\rm{one}}\,{\rm{s + three}}\,{\rm{p}}\,{\rm{ + }}{{\rm{d}}_{{\rm{xy}}}}{{\rm{d}}_{{\rm{yz}}}}{{\rm{d}}_{{\rm{xz}}}}\)		Pentagonal bipyramidal	\({\rm{I}}{{\rm{F}}_{\rm{7}}}\)

Predicting Hybridisation of Central Atom and Shape of Molecules

The prediction of the number of hybrid orbitals \(\left( {\rm{X}} \right)\) can be made by a very simple formula:

\(x = \)

\(1/2\) (Number of valence electrons of the central atom) + (Number of monovalent atoms or groups of atoms surrounding central) – (Charge of cation if the species is polyatomic cation) + (Charge of anion if the species is polyatomic anion)}

Further, the value of \({\rm{X}}\) can be used to predict the hybridisation of the central atom of a molecule by,

Value of \(x\)	\(2\)	\(3\)	\(4\)	\(5\)	\(6\)	\(7\)
Type of Hybridisation	\({\rm{sp}}\)	\({\rm{s}}{{\rm{p}}^{\rm{2}}}\)	\({\rm{s}}{{\rm{p}}^{\rm{3}}}\)	\({\rm{s}}{{\rm{p}}^{\rm{3}}}{\rm{d}}\)	\({\rm{s}}{{\rm{p}}^{\rm{3}}}{{\rm{d}}^{\rm{2}}}\)	\({\rm{s}}{{\rm{p}}^{\rm{3}}}{{\rm{d}}^3}\)

Also, the prediction of shape can be made by observing the number of surrounding atoms and the number of lone pairs on an atom. If the number of surrounding atoms is equal to the number of hybrid orbitals, the regular shape of the hybrid molecule takes place.

However, suppose the number of surrounding atoms are less than the hybrid orbitals. In that case, the difference gives the lone pair on the central atom giving rise to the irregular shape of the molecule.

FAQs

Q.1. What are the types of Hybridisation?
Ans: Hybridisation is of multiple types depending on the number of orbitals and the types of orbitals involved in the formation of hybrid orbitals.

Type of Hybridisation

\({\rm{sp}}\)

\({\rm{s}}{{\rm{p}}^{\rm{2}}}\)

\({\rm{s}}{{\rm{p}}^{\rm{3}}}\)

\({\rm{s}}{{\rm{p}}^{\rm{3}}}{\rm{d}}\)

\({\rm{s}}{{\rm{p}}^{\rm{3}}}{{\rm{d}}^{\rm{2}}}\)

\({\rm{s}}{{\rm{p}}^{\rm{3}}}{{\rm{d}}^{\rm{3}}}\)

The shape of the molecule formed after this hybridisation is tetrahedral, having an angle of \({\rm{10}}{{\rm{9}}^{\rm{o}}}\,{\rm{28′}}{\rm{.}}\) The hybrid orbitals formed have approximately \({\rm{10}}{{\rm{9}}^{\rm{o}}}\,{\rm{28′}}{\rm{.}}\) character and \({\rm{75\% }}\,{\rm{p}}\) character.

For example, while the bond formation of \({\rm{C}}{{\rm{H}}_{\rm{4}}}{\rm{,}}\)

\({\rm{C = 1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1{\rm{2p}}_{\rm{z}}^0\) (Ground State)

\({\rm{C}}\,{\rm{ = }}\,{\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_{\rm{x}}^{\rm{1}}{\rm{2p}}_{\rm{y}}^{\rm{1}}{\rm{2p}}_{\rm{z}}^{\rm{1}}\)
(Excited State) 

Q.4.What is the purpose of hybridisation?
Ans: The purpose of hybridisation is to understand the bond formation of different types of molecules and the shapes of the molecules by intermixing different orbitals of similar energy levels and providing the basic logic behind the shapes of molecules around.

So when four hydrogen atoms arrive for bond formation, they make a bond with four \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybrid orbitals made after intermixing of \({\rm{2}}{{\rm{s}}^{\rm{1}}}{\rm{2p}}_{\rm{x}}^1{\rm{2p}}_{\rm{y}}^1{\rm{2p}}_{\rm{z}}^1\) orbitals and have a tetrahedral shape.