Hybridization

Linus Pauling introduced the concept of hybridization in order to provide a simple and effective explanation for the characteristic geometrical shapes of polyatomic molecules. Hybridization is a process in which atomic orbitals combine to create a set of new hybrid orbitals. This involves the blending or mixing of individual atomic orbitals, resulting in a combination with characteristics distinct from the original orbitals.

1.0Need For Hybridization

This concept has been instrumental in understanding and predicting the structures of various molecules, such as CH₄ (methane), NH₃ (ammonia), H₂O (water), and many others. It helps bridge the gap between theoretical quantum mechanics and experimental observations, making it easier to comprehend the arrangement of atoms in complex molecules. Let’s discuss how the concept of hybridization arose:

Observation of Molecular Geometry:

• Experimental observations of molecular geometry did not always align with the expected geometry based solely on the arrangement of atomic orbitals.

Molecular Orbital Theory:

• Molecular Orbital Theory, based on quantum mechanics, provided a better understanding of molecular structure by combining atomic orbitals to form molecular orbitals. However, this theory didn't fully explain certain molecular geometries.

Valence Bond Theory and Hybridization:

• Linus Pauling introduced the Valence Bond Theory, which aimed to combine the best features of atomic and molecular orbital theories. He proposed that atomic orbitals could mix or hybridize to form new, hybrid orbitals that are better suited to describe the bonding in molecules.

Hybrid Orbitals:

• Hybrid orbitals result from the mixing of atomic orbitals, combining their unique shapes and energies for the formation of bonds. These hybrid orbitals are then used to form sigma bonds with other atoms. The most common types of hybridization involve s and p orbitals and result in sp, sp², and sp³ hybrid orbitals.

2.0What is Hybridization

• Mixing of different shapes and approximate equal energy atomic orbitals and redistribution of energy to form new orbitals of the same shape and same energy. These new orbitals are called hybrid orbitals, and the phenomenon is called hybridization.
• Central atom undergoes hybridisation.
• This process takes place before bond formation.
• In hybridisation fulfilled, half-filled and empty orbitals can participate.
• The number of hybrid orbitals formed is equal to the number of atomic orbitals that participate in hybridisation.
• Each hybrid orbital has two lobes; one is major and other is minor. The bond will be formed from the major lobe.
• The directional properties in hybrid orbitals are more than atomic orbitals. Therefore, hybrid orbitals form stronger sigma bond.
• Hybrid orbital should be arranged in such a way that repulsion between hybrid orbital should be minimum.

3.0Importance of Hybridization

Features and Characteristics of hybrid orbitals:

1. Equal Energy and Characteristics:

• Hybrid orbitals of equal energy are formed by mixing a specific number of atomic orbitals. For example, mixing one s orbital and three p orbitals creates four sp³ hybrid orbitals, each with identical characteristics. These hybrid orbitals have a distinct mix of s and p (or d) orbital character, influencing their shape and energy.

2. Shape of Hybrid Orbitals:

• Each hybrid orbital has a characteristic shape where one lobe is larger than the other. This shape arises due to the directional nature of atomic orbitals and the mixing process that forms hybrid orbitals. For instance, sp³ hybrid orbitals have a tetrahedral shape with one larger lobe and one smaller lobe.

3. Effect of Orbital Character on Shape and Size:

• The percentage of s, p, or d orbital character in a hybrid orbital affects its size and shape. Higher s-character makes the hybrid orbital bulkier and shorter, while higher p- or d-character makes it longer and thinner. This relationship is crucial for understanding molecular geometry and bond angles.

4. Energy of Hybrid Orbitals:

• The energy of a hybrid orbital correlates with its orbital character composition. Higher s-character lowers the energy of the hybrid orbital, whereas higher p- or d-character increases its energy. This energy variation influences molecular stability and reactivity.

Percentage of s, p, d characters in various hybridized orbitals:

4.0Hybrid Orbitals and Geometries

Hybridization type	Geometry	Representation
sp	Linear
sp2	Trigonal Planar
sp3	Tetrahedral
sp3d	Trigonal Bipyramidal
sp3d2	Octahedral
sp3d3	Pentagonal Bipyramidal

5.0Types of Hybridization

Let’s understand What are the different types of hybridization in detail:

1. sp Hybridization: 

sp hybridization involves the mixing of one s orbital and one p orbital to form two sp hybrid orbitals. These orbitals are linearly oriented and are involved in the formation of triple bonds. Examples include molecules like acetylene (C₂H₂) and carbon dioxide (CO₂), BeH₂, BeF₂, BeCl₂, BeBr₂, BeI_2.

2. sp² Hybridization: 

In sp² hybridization, one s orbital and two p orbitals mix to form three sp² hybrid orbitals. These orbitals are trigonal planar in shape and are involved in the formation of double bonds. Examples include molecules like ethylene (C₂H₄) and boron trifluoride (BF₃), Graphite, HNO₃, SO_3. 

3. sp³ Hybridization: 

This type of hybridization involves the mixing of one s orbital and three p orbitals to form four sp³ hybrid orbitals. These orbitals are tetrahedrally oriented around the central atom. Examples include molecules like methane (CH₄) and ethane (C₂H₆), CCl₄, CBr₄.

4. sp³d hybridization:

In sp³d hybridization, one s, three p, and one d orbital combine to form five sp³d hybrid orbitals. These orbitals are arranged in a trigonal bipyramidal shape. They have the same energy level and participate in sigma bond formation. Examples include PF₅, PCl₅, PBr₅, AsF₅, AsCl₅, SbCl₅.

5. sp³d² hybridization: 

In sp³d² hybridization, one s orbital, three p orbitals, and two d orbitals from the same shell of an atom combine to form six hybrid orbitals. These hybrid orbitals are arranged in an octahedral geometry around the central atom, with bond angles of 90° between them. Examples include SF₆, AlF₆^3⊖ , ICl₅,  XeOF₄

6. sp³d³ hybridization:

In sp³d³ hybridization, one s orbital, three p orbitals, and three d orbitals combine to form seven hybrid orbitals. These sp³d³ orbitals are arranged in a pentagonal bipyramidal shape, with five bond angles of approximately 72° and two bond angles of 90°. This type of hybridization is observed in molecules with pentagonal bipyramidal geometry, playing a key role in predicting their structure and properties.

Examples include- IF₇, XeF₆

7. dsp³ Hybridization: 

This type of hybridization involves the mixing of one s orbital, three p orbitals, and one d orbital to form five dsp³ hybrid orbitals. These orbitals are used in molecules with square pyramidal or trigonal bipyramidal geometries. Examples include molecules like Iron pentacarbonyl, [Fe(CO)5].

8. d²sp³ Hybridization: 

d²sp³ hybridization occurs when one s orbital, three p orbitals, and two d orbitals mix to form six d²sp³ hybrid orbitals. These orbitals are involved in molecules with octahedral geometries. Examples include molecules like [Co(NH₃)₆] complex.