Hybridisation : Definition, Types ,Rules & FAQs

Hybridisation

Have you ever wondered why different molecules or compounds show different shapes? Why are the bonds between the central atom and other atoms formed in a certain manner? What is the concept that changes the geometry and spatial arrangement of molecules?

It is critical in arranging different molecules’ shapes, energy, etc. Now, what is hybridisation, and how does it arrange the bonds? This article will answer these questions.

What is Hybridisation?

It is the hypothesis of mixing atomic orbitals to form advanced hybrid orbitals appropriate for describing the bonding properties. It leads to the entirely new shape and orientation of the valence orbitals of an atom.

It is essentially an attempt to predict where the electrons are when there are bonds and lone pairs. It occurs when atomic orbitals mingle to form new atomic orbitals. The advanced orbitals have equal electron capacity as the old ones. The energies and properties of the new and hybridised orbitals are an ‘average’ of the original unhybridised orbitals.

The need to introduce the concept of hybridisation was the finest explanation for the certainty that all of the C-H bonds in molecules such as methane were indistinguishable. 

Concept of Hybridisation

Linus Pauling introduced the concept of hybridisation. This concept describes the characteristic dimensional shapes of molecules with more than two atoms like CH₄, H₂O, NH₃, etc. He suggested that the atomic orbitals integrate to form a new set of identical orbitals recognised as hybrid orbitals. Different from pure orbitals, intermixed orbitals are utilised in forming the bond. Such an episode is recognised as hybridisation. This process can be the intermixing of the orbitals of slightly different energies to redistribute them, resulting in the formation of a new set of orbitals of equivalent energies and shapes.

Types of Hybridisation

There are different types of hybridisation involving s, p and d orbitals. The different types are as under:

1. sp hybridisation:

• One p and one s orbital blend, forming two sp composite (or hybrid) orbitals.
• They are equivalent and symmetrical hybrid orbitals.
• Each sp hybrid orbitals has equal character, i.e., 50% s-character and 50% p-character.
• The molecules formed are linear in geometry and thus have a bond angle of 180°.
• s-orbital is spherical, and p-orbital is pear-shaped, but the sp hybrid orbital, obtained from the mixing of s and p orbital, is oval-shaped.
• This type of hybridisation is also known as diagonal hybridisation, i.e., spatial arrangement is directed in a straight line in opposite directions.
• Example: BeCl₂, BeF₂, C₂H₂, BeH₂, etc.

2. sp² hybridization:

• One s and two p orbitals blend, forming three sp² fused (or hybrid) orbitals.
• It is also called trigonal hybridisation.
• They have the same shape and same energy.
• They lie in one plane.
• Each sp² hybrid orbital has 33% s-character and 66% p-character.
• The molecules formed are triangular planar in geometry and thus have a bond angle of 120°.
• sp² hybrid orbital is obtained from mixing s and two p orbitals. Hence, it has a trigonal spatial arrangement.
• As the contribution of the p-orbital is more than the s-orbital, the shape of sp² hybrid orbitals, therefore, tends more towards a pear.
• Example: BeH₃, BeF₃, etc., and alkenes

3. sp³ hybridisation:

• One s and three p orbitals intermix to form four sp³ composite (or hybrid) orbitals.
• It is also called tetrahedral hybridisation.
• sp³ hybrid orbitals are equivalent and symmetrical hybrid orbitals.
• These have the same shape and same energy.
• Each sp³ hybrid orbital has one-fourth s-character and three-fourth p-character, i.e., 25% s-character and 75% p-character.
• The molecules formed are tetrahedrons in geometry and thus have a bond angle of 109.5°. It is also called a tetrahedral angle.
• Example: H₂O, H_,2S, C₂H₆, SiCl₄, CH₄, etc.

4. sp³d hybridisation:

• One s, three p, and one d orbital intermix and form five sp³d intermix (or hybrid) orbitals.
• It is also called trigonal bipyramidal hybridisation.
• These have the same shape and same energy.
• The molecules formed are trigonal bipyramidal in geometry and thus have a bond angle of 120° in the horizontal plane and 90° in the vertical plane.
• They are directed toward the five corners of a regular pentagonal bipyramid, i.e., the arrangement of the five hybrid orbitals in space is pentagonal bipyramidal. 
• Example: PCl₅, SbF₅, ClF₃, XeF₂, etc.

5. sp³d² hybridisation:

• One s, two d, and three p orbitals intermix to form six sp³d²hybrid orbitals.
• They have the same shape and same energy.
• The molecules formed are octahedrons in geometry and thus have a bond angle of 90°.
• They are directed toward the six corners of a regular (symmetrical) octahedron, i.e., the octahedral arrangement of the six hybrid orbitals in space.
• Example: SF₆, SeF₆, IF₅, XeF₄, etc.
• 

6. sp³d³ hybridisation:

• One s, three p, and three-d orbitals intermix to form six sp³d³ hybrid orbitals.
• They have the same shape and same energy.
• They are directed toward the seven corners of a regular (symmetrical) pentagonal bipyramidal, i.e., the arrangement of seven hybrid orbitals in space is the pentagonal bipyramidal.
• The molecules formed are trigonal bipyramidal in geometry and thus have a bond angle of 120° in the horizontal plane and 90° in the vertical plane.
• The sum of 𝜎-bond pairs and lone pairs in the species showing sp³d³ hybridisation is always equal to 7.
• Example: IF₇, XeF₆, ReF₇, etc.

The different types of hybridisations in a chart are given below:

Molecule Type	Number of electron pairs	Geometry	Bond Angle	Hybridisation	Example
AB2	2	Linear	180°	sp	BeCl2
AB3	3	Trigonal planar	120°	sp2	BF3
AB4	4	Tetrahedral	109.5°	sp3	CH4
AB5	5	Trigonal bipyramidal	90°, 120°	sp3d	PCl5
AB6	6	Octahedral	90°	sp3d2	SF6
AB2E	3	Bent	120° (119°)	sp2	SO2
AB3E	4	Trigonal bipyramidal	109.5° (107.5°)	sp3	NH3
AB2E2	4	Bent	109.5° (104.5°)	sp3	H2O
AB4E	5	See-saw	180°, 120° (173.1°, 101.6°)	sp3d	SF4
AB3E2	5	T-shape	90°, 180° (87.5°, <180°)	sp3d	ClF3
AB2E3	5	Linear	180°	sp3d	XeF2
AB5E	6	Square pyramidal	90° (84.8°)	sp3d2	BrF5
AB4E2	6	Square planar	90°	sp3d2	XeF4
AB7	7	Pentagonal bipyramidal	90°, 180°	sp3d3	XeF6

Need of Hybridisation

Let’s take, for example, the BeCl₂ molecule to understand the need for hybridisation.

The ground state configuration of Beryllium (Be) is 1s² 2s².

So, to form two bonds, it gets into an excited state,

Be- 1s² 2s¹ 2p¹

In this state, Be can form bonds with two Cl, but the bonds would not be equal (identical). It is because one bond will consist of the spherical 2s orbital of Be overlapping with a dumb-bell-shaped 3p orbital of Cl. In contrast, the other bond will consist of the dumb-bell-shaped 2p orbital of Be overlapping with the dumb-bell-shaped orbital of Cl.

But the actual shape of the BeCl₂molecule is linear and equal. It means the bond lengths and energies for both Be-Cl bonds are identical. This contradiction was resolved by hybridisation theory. Therefore, it raises the need for hybridisation in molecules.

Conclusion

From the above discussion, you are now well-versed with the questions like what is hybridisation, different types, charts, etc. It is a very convenient way to explain chemical bonding between different atoms.

In short, it is the phenomenon of mixing two or more degenerate atomic orbitals to form a new set of orbitals. And to remember different types of hybridisations, and a chart is the best possible way.

Frequently Asked Questions

Q1. What are the salient features of hybridisation?

Answer: The main features of hybridisation are as under:

1. The number of hybrid orbitals formed equals the total number of pure atomic orbitals that get hybridised.
2. Atomic orbitals can mix effectively to give hybrid orbitals only when they are of the almost same energy and same symmetry.
3. The hybrid orbitals are more successful in forming stable 𝜎-bonds than the pure atomic orbitals (𝜋-bond).
4. These hybrid orbitals are governed in space in specific directions around the central atom. This arrangement stabilised the geometry by minimising the repulsion between electron pairs. Therefore, the type of hybridisation indicates the spatial orientation of the molecules.
5. Only the 𝜎-bond pairs and lone pairs of electrons decide the molecule’s shape. 𝜋-bond does not participate in it.

Q2. What are vital conditions for hybridisation?

Answer: The vital conditions for hybridisation are:

1. The orbitals present in the atom’s valence shell are hybridised.
2. The orbitals that encounter hybridisation should have virtually equal energy.
3. The advancement of electrons is not an essential condition before hybridisation.
4. Only half-filled orbitals don’t need to participate in hybridisation. In some cases, even filled valence shell orbitals take part in it.

Q3. What are the factors that decide the hybridisation of a molecule?

Answer: Three key factors decide the hybridisation of a molecule. These are:

• The number of 𝜎-bonds: 𝜎-bonds play an important role in the shape formation of a molecule. These bonds are strong than 𝜋-bonds.
• The number of lone pairs: Lone pairs decide the spatial orientation of a molecule. For example, in the case of H₂O, the hybridisation is sp³. But due to lp-lp repulsion, the bond angle reduced from 109.5° to 104.5°.
• 𝜋-bonds arise from unhybridised orbitals.

Q4 What is Orbital hybridisation? 

Answer: According to the valence bond theory, orbital hybridization is the process of combining atomic orbitals to create new hybrid orbitals that are ideal for pairing electrons to form chemical bonds.