Hybridization is intermixing of atomic orbitals of different energy and shapes to form new hybrid orbitals. The new hybrid orbitals have the same energy and shape.
Linus Pauling introduced the phenomenon of hybridization to explain molecular geometries. He pointed out that in methane molecules four atomic orbital carbon atoms are involved in bonding. That is the reason there is no pi-bond distribution in methane.
There are different types of hybridization depending upon the number and nature of orbitals taking part in hybridization such as sp, sp2, sp3, dsp2, sp3d, sp3d2, and sp3d3.
Hybridization is a concept of mixing two or more non-equivalent atomic orbitals of comparable energy and different shapes. Their energy is always lower than the energy of parent orbitals. Hence hybrid orbitals are more stable as compared to the parent atomic orbitals. For example, atomic orbitals s and p and their hybridization process are given below:
In simple words, hybridization is the description of observed molecular geometry and electron density. Because of hybridization molecules adopt a particular shape to overcome repulsive interactions. In addition, it tells how molecular geometry is affected by ligands and electrons in a particular atom.
Key features of the Hybridization process
- Hybridization is the theoretical model to explain covalent bonding.
- For hybridization, there must be two or more nonequivalent orbitals to be mixed to form a single hybrid orbital.
- It involves unpairing and promotion of the electrons to the next available orbital. This is called the excitation of electrons.
- Excited orbitals then undergo hybridization.
- The number of hybrid orbitals is equal to the number of atomic orbitals mixed.
- The newly formed orbital has the same energy and shape.
- The hybrid orbitals do not exist in free form.
- Not only do half-filled orbitals undergo hybridization, but completely filled and empty orbitals can also undergo hybridization.
- Bond formation occurs when hybrid orbitals overlap with suitable orbitals of other atoms.
- Atomic orbitals of the same energies can undergo hybridization and form stronger bonds.
Types of hybridization
The types of hybridization depend upon the number and kinds of atomic orbitals involved in hybridization. When only s and p orbitals are involved then, sp, sp2, and sp3 are the resultant hybrid orbitals. On the other hand, when s, p, and d orbital are involved then, dsp2 sp3d sp3d2 and sp3d3 hybrid orbitals.
When one s and one p-orbital combine together to form two hybrid orbitals of the same shape and energy then it is called sp hybridization. There is an angle of 180º between two hybrid orbitals. Each sp hybrid orbital has 50% s-character and 50% p-character. Further, these two hybrid orbitals lie in one plane oriented in opposite directions.
For example, ethyne, BeH2, HgX2, BeF2, CO2, and BeCl2 have sp hybridization of the central atom.
Hybridization of BeCl2
One s and one p orbital of beryllium are involved in hybridization to form two sp orbitals. These sp orbitals form two sigma bonds with Cl.
When one s and two p-orbitals on the same atom intermix to form three identical sp2 hybridized orbitals it is called sp2 hybridization. These sp2 hybrid orbitals have the same energy and shape. All the hybrids lie in one plane. The bond angle between any two orbitals is 120º and all the bonds are directed toward the corners of the triangular plane.
Note that, sp2 hybridized molecules have a trigonal planar geometry. In addition, in sp2 hybridized orbitals, there will be 33% s-character and 66% p-character.
Hybridization of BCl3
In boron trichloride, boron is the central atom bonded to three chlorine atoms. In the ground state, boron has one unpaired electron. It promotes one of its 2s electrons to the empty p orbital. Now it has three unpaired electrons in three half-filled orbitals. Thus, boron undergoes sp2 hybridization by using one 2s and two 2p orbitals as follows:
BF3, BH3, SO3, SO2, and PbCl2
sp3 hybridization involves the mixing of one ‘s’ and three p-orbitals on the same atom to form four identical sp3 hybrids. All the sp3 hybrids are directed at the corners of a regular tetrahedron. In addition to it, sp3 hybridized molecules exhibit tetrahedral geometry bond angle between any two bonds is 109.5º. It is to be noted that in sp3 hybridization, there will be 25% s-character and 75% p-character.
For example, in the CH4 molecule, the carbon atom undergoes sp3 hybridization. Carbon atom the central atom has two unpaired electrons in p-orbitals in the ground state. During excitation, one of the 2s electrons is promoted to the p orbital. Now it has four half-filled orbitals available for bonding. By mixing one 2s and three 2p orbitals to form four sp3 hybrid orbitals.
It is of keen importance that all sp3 hybrid orbitals form a sigma bond and the bond angle between any two bonds is 109.5º.
Further examples of molecules having sp3 hybridization are H2O, PF3, and PCl3.
In this type of hybridization, the d-orbital is also involved. It is the mixing of one ‘s’ two p and one d-orbitals to form four identical dsp2 hybrid orbitals. All four hybrid orbitals are oriented to the corners of a square. This is the reason why dsp2 hybridized molecules have square planar geometry. It is mostly encountered in coordination compounds because of the d-orbital.
For example, nickel shows dsp2 hybridization in [Ni(CN)4]2- . Nickel cyanide ion is a diamagnetic molecule having valence shell electronic configuration 3d8, 4s2. In Ni+2 valence shell electronic configuration is 3d8, 4s0. Considering this, the empty 3d, 4s, and 4p orbitals undergo dsp2 hybridization and make bonds with cyanide (CN–) ligands.
In this type of hybridization one s, three p, and one d-orbitals are involved to form five hybrid orbitals. All sp3d hybrids are oriented toward the corners of a trigonal bipyramid. They form three coplanar bonds having a bond angle of 120º, while the other two bonds are at a right angle, one above and the other below the plane.
It is a nonpolar molecule because both axial bonds cancel the dipole moment. All five bonds are not equivalent. The three orbitals are directed trigonally and are called equatorial orbitals. While the other two are known as linear orbitals.
Further examples of sp3d
SF4, PCl5(g), XeF2, PBr5, AsCl5, ClF3, I3, IF2, etc.
In this type of hybridization one s, three p, and two d-orbitals are involved in the hybridization. They give six equivalent hybrid orbitals. These orbitals are oriented toward the corner of the octahedron. The bond angle between any two bonds is 90º.
For instance, in a sulfur hexafluoride molecule, sulfur is the central atom having two unpaired electrons in a 3p orbital. In order to bond with six fluorine atoms, it requires six half-filled orbitals. For this purpose, it promotes one of 3s electron and one of 3p electron to two empty d orbitals. Now it has 6 unpaired electrons in an excited state. These orbitals hybridized to give six sp3d2 hybrid orbitals. Additionally, these hybrid orbitals are directed to the corner of a regular octahedron.
Examples of sp3d2:
SF6, IF5, XeF4, PBr5, AsCl5 etc.
In this type of hybridization one s, three p, and three d atomic orbitals are involved in the hybridization. They give seven nonequivalent hybrid orbitals. Five hybrid orbitals are pointed toward the vertices of the pentagon they have a bond angle of 72º, while the remaining two are perpendicular to the plane. In addition to it, they have a bond angle of 90º. Furthermore, These orbitals are oriented toward the corner of the pentagonal bipyramid.
Hybridization of IF7
Iodine is the central atom in iodine heptafluoride, having one unpaired electron in a 5p orbital. It promotes one of 5s electrons and two of 5p electrons to three empty 5d orbitals to get 7 unpaired electrons in an excited state.
These orbitals are hybridized to give seven sp3d3 hybrid orbitals. Moreover, These hybrid orbitals are directed to the corner of the regular pentagon.
Who introduced the concept of hybridization?
The concept of hybridization was proposed by L. Pauling in 1931 to explain the tetrahedral geometry of methane-type compounds.
What is meant by molecular geometry?
Molecular geometry is the three-dimensional arrangement of atoms in a molecule. It can be determined by the central atoms, ligands, and electron pairs.
What are degenerate orbitals?
Atomic orbitals having the same energy levels are called degenerate orbitals. An atom has s,p,d, and f subshells. The p orbital has three degenerate orbitals px, py, and pz.
What are hybrid orbitals?
The orbitals which are formed by the mixing of two or more atomic orbitals of different energy and shape are known as hybrid orbitals.
Does VBT explain the resonance structure of molecules?
VBT can explain the resonance structure of molecules. it can represent the full electronic structure of molecules.
What are the drawbacks of VBT?
- This theory is unable to explain the difference between square planar and tetrahedral geometry.
- It is unable to explain the paramagnetic behavior of O2.
- VBT cannot explain the non-existence of noble gas molecules.
- It is unable to explain the structures of odd electron molecules or ions where no pairing of electrons occurs.
- The second edition of Inorganic Chemistry by Catherine E. Housecroft and Alan G. Sharpe
- Caravan Advanced Inorganic Chemistry by Haq Nawaz Bhatti.
- The tenth edition of Chemistry by Raymond Chang.
- Orbital hybridisation by (en.wikipedia.org)