Atomic Orbital Hybridization

Atomic Orbital Hybridization

Definition of Hybridization

In order to account for equivalent tetravalency of carbon, trivalency of group III A elements, divalency of group II A elements, identical bonds formation by group IVA (C), III A and IIA elements and to explain bond angles in H₂O and NH₃ molecules, Linus Pauling introduced the concept of hybrid orbitals and hybridization. The process of hybridization is a hypothetical process and for an isolated atom hybridization has no meaning. The concept of hybridization is an extension of atomic orbital theory (AOT) or valence bond theory (VBT).

The word hybridization means mixing or blending. The process of blending or mixing of different pure atomic orbitals of an atom differing slightly in energy to produce a set of new equivalent orbitals of identical energy, size and shape called hybrid orbitals which are equal in number to the mixing orbitals having specific spatial orientation in space round the atom is termed as atomic orbital hybridization and the new equivalent blended or modified atomic orbitals formed having same energy, size and shape possessing specific geometry are called hybrid orbitals. Hybrid orbitals are also atomic orbitals but are not pure atomic orbitals and are regarded as modified or blended atomic orbitals. The number of Hybridized orbitals produced is equal to the number of hybridizing atomic orbitals (which are being hybridized).

Mixing of a pure s-orbital and one or more p-orbitals is rather like mixing of a gallon of pure red pain and one or more gallons of white paint to give two or more gallons of new pink coloured paint.

Significance of Hybridization

The concept of atomic orbital hybridization gives a satisfactory explanation for the equivalent valencies of the polyvalent elements. It holds significant importance in determining the nature of bonds and shape (geometry) of the polyatomic molecules.

Conditions for Hybridization

1.        The orbitals of same atom take part in hybridization.

2.        Hybridizing orbitals should have small difference in their energies.

3.        Only the orbitals and not the electrons get hybridized.

Characteristics of Hybrid Orbitals

1.   The hybrid orbitals are of equivalent shape, size and energy.

2.   The number of hybrid orbitals produced is equal to the number of hybridizing orbitals.

3.  Hybrid orbitals are also atomic orbitals.

4. They show properties and energies intermediate between those of parent atomic orbitals.

Types of Hybridization

1.  sp³- hybridization                       

2.  sp²- hybridization                       

3.  sp- hybridization

Explanation of orbital hybridization with special reference to Carbon and Boron

Ground State Configuration

The number of half-filled valence orbitals or unpaired electrons in the valence shell of an atom constitutes its valency. Boron and Carbon from their ground or atomic states electronic configuration appear to be monovalent and divalent respectively due to the presence of one and two half-filled orbitals respectively. But boron exhibits trivalency (i.e. shows 3 valency) and carbon shows tetravalency in most of its compounds (organic compounds).

Excited State Configuration

To account for these discrepancies (anomalies), it is assumed that one of the electron from the lower energy orbital belonging to the ground state (2s) is promoted to empty p_y and p_z orbital achieving the excited state. The energy required for the excitation (promotion) and unpairing the electron (of 2s) is compensated by the energy released during hybridization and the process of covalent bond formation. The excited state electronic configuration results in an increase in the number of unpaired electrons. According to these excited state electronic configurations, boron becomes trivalent while carbon becomes tetravalent.

Hybridized State

On the basis of excited state electronic configuration, it might be expected that carbon would form 4 (and boron would form 3) covalent bonds. One expect that in CH₄, three C – H bonds formed by the overlap of three 2p–orbitals would be identical (having directional nature and higher energy) while the fourth C – H bond formed due to 2s-orbtial would be different (having non-directional nature and lower energy). In actual practice, all four C – H bonds in CH₄ are identical in all respects (i.e. in bond energy and bond length).

It means that atomic orbitals of carbon have equalized their energies, size and shape. This process is called Hybridization and carbon is said to be HYBRIDIZED.

 sp³-Hybridization/Tetragonal Hybridization

Definition

The type of hybridization in which one s (2s) and three p (2p) atomic orbitals get mixed to produce four new equivalent sp³ hybrid orbitals which are arranged tetrahedrally with bond angle 109.5 is called sp³ Hybridization.

Energy Level Diagram

Character

1.        Each sp³ hybridized orbital has 25% s-character and 75% p character (1:3).

2.        Each sp³ hybrid orbital has 1 electron.

3.     sp³-hybrid orbitals are four in number, which are arranged tetrahedrally with carbon located at  the centre. Angle between two sp³-hybrid orbital is 109.5^○ (109.28^○).

Occurrence of sp³-hybridization

sp³ hybridization is found in those compounds where an atom (carbon) is bonded by other 4 atoms. e.g. Alkane i.e. Methane (CH₄), Ethane (C₂H₆), CCl₄, SiCl₄, SnCl₄ etc.

Hybrid Molecular Structure of Methane

In methane i.e. CH₄, carbon is bonded to four hydrogen atoms, thus carbon gets sp³-hybridized. Each H – C bond is sigma bond which is formed due to s-sp³ overlapping. There are four sigma bonds in CH₄. Thus CH₄ has tetrahedral geometry. Each bond angle in CH₄ is 109.28^○ (109.5^○). The bond length between C–H is 1.09^○A.