Hybrid orbitals are degenerate orbitals formed by mixing of orbitals differing in small amounts with each other in energy. The concept of hybridization allows the "construction" of new orbitals on atoms, so that the bonding in a molecule is made consistent with its known geometry.
Hybrid orbitals will always overlap head to head to form sigma bonds. The orbitals that overlap to form the sigma bonds must overlap head to head or end to end. By assuming the molecular geometry one can identify hybrid orbitals.
DefinitionBack to Top
Hybridization may be defined as "the phenomenon of mixing of atomic orbitals of nearly equivalent energy, involving redistribution of energy, to form new orbitals of equal energy known as hybrid orbitals." The number of hybrid orbitals is equal to the number of the orbitals hybridized. The properties of the hybrid orbitals are in between the properties of the orbitals which are hybridized.
Hybridization may precisely define as the phenomenon of mixing up of orbitals of an atom of nearly equal energy, giving rise to entirely new orbitals equal in number to the mixing orbitals and having same energy contents and identical shapes.
TypesBack to Top
- Since hybridization leads to an entirely new shape and orientation to the valence orbitals of an atom, it plays a significant role in determining the shape and geometry of molecules formed from such orbitals.
- There are various types of hybrid orbitals, depending upon the number and nature of the orbitals undergoing hybridization.
- The number of orbitals is always conserved in hybridization; that is a given number of atomic orbitals hybridize to form an equivalent number of hybrid orbitals.
- One 2s orbital and three 2p orbitals form four sp3 hybrid orbitals.
- One 2s orbital and two 2p orbitals form three sp2 hybrid orbitals.
- One 2s orbital and one 2p orbital form two sp hybrid orbitals.
ExamplesBack to Top
In BeH2, the Be atom is sp hybridized because it is surrounded by two groups. Each Be-H bond is formed by overlap of an sp hybrid orbital from Be and a 1s orbital from H. The sp hybrid orbitals are oriented 180o away from each other.
2. sp2 hybrid orbitals
In BF3, the B atom is sp2 hybridized because it is surrounded by three groups. Each B-F bond is formed by overlap of an sp2 hybrid orbital from B and a 2p orbital from F. The sp2 hybrid orbitals all lie in a plane, and are oriented 120o apart. The B atom also has a vacant unhybridized 2p orbital. This orbital is located above and below the plane of the BF3 molecule.
3. sp3 hybrid orbitals
The N atom in NH3 and the O atom in H2O are both surrounded by four groups, making them sp3 hybridized. Each N-H and O-H bond in these molecules are formed by overlap of an sp3 hybrid orbital with a 1s orbital from H. The lone pairs of electrons on N and O also occupy an sp3 hybrid orbital.
Hybrid Orbitals ChartBack to Top
The types of hybrid orbitals with their bond angle, example and observed bonding is tabulated below.
|Number of groups bonded to C||Hybridization||Bond angle||Example||Observed bonding|
Atomic OrbitalBack to Top
Atomic orbitals represent regions in space in which a particle of particular energy is most likely to be found. The state of an atom can be determined by four quantum numbers, principal, angular, magnetic and spin quantum numbers.
Of atomic orbitals s-orbitals are spherically symmetric, p-orbitals are distributed orthogonally in space and d-orbitals comprise three 2tg and two eg orbitals. Combination of atomic orbitals leads to molecular orbitals and chemical bonds. Single bonds are due to the combination of s and p orbitals forming cylindrically symmetric $\sigma$ bonds. A combination of $\sigma$ and $\pi$ bonds leads to the formation of double bonds. The aromatic nature of planar molecules like benzene is due to the $\pi$-bonding in these molecules, with $\pi$-electron clouds distributed above and below the plane of the molecule.
The shapes of s, p, d and f orbitals are shown below.
SP Hybrid OrbitalsBack to Top
An sp hybrid orbital is composed of 50% p orbital and 50% s orbital. Therefore its energy is halfway between the energies of the s orbital and the p orbital. In forming sp hybrid orbitals, two equivalent electrons that can form sigma bonds. The two remaining unhybridized p electrons can overlap with p electrons from the oxygen atoms to form the required double bonds.
For example, carbon dioxide O=C=O has two sigma bonds and two pi bonds. The hybridization for this molecule is shown below.
Hybridization of carbon to produce the sp hybrid orbitals. The two unhybridized p electrons are available to form pi bonds.
SP2 Hybrid OrbitalsBack to Top
The energy of an sp2 orbital is higher than that of the s orbital by 67% of the difference between the energies of the p orbital and the s orbital. This hybridization involves mixing of one orbital of s-sub level and two orbitals of p-sub level of the valence shell to form three sp2 hybrid orbitals. These sp2 hybrid orbitals lie in a plane and are directed towards the corners of an equilateral triangle.
Each sp2 hybrid orbital has one third s-character and two third p-character. sp2 hybridization is also called trigonal hybridization. The molecules in which central atom is sp2 hybridized and is linked to three other atom directly have a triangular planar shape.
SP3 Hybrid OrbitalsBack to Top
The energy of an sp3 orbital is higher than the energy of the s orbital by 75% of this difference. The mathematical combination of the 2s atomic orbital and three 2p atomic orbital forms four equivalent sp3 hybrid orbitals. Atoms with four sp3 hybrid orbitals are referred to as sp3 hybridized, or as having a hybridization state of sp3 and are directed towards the corner of the tetrahedral.
The energy of the electron in the hybrid orbitals lies between the original s and p energies. When the sp3 hybrid orbitals form, their orientation is tetrahedral. Overlap of the four identical sp3 electrons with electrons from hydrogen atoms forms the tetrahedral methane molecule. Any molecule whose basic structure is the tetrahedron will have sp3 hybrid orbitals.
Determining Hybridization of AtomsBack to Top
A simple method for determining hybridization in carbon compounds is by determining how many atoms are attached to the carbon atom. If two atoms are attached, the hybridization is sp, if three then sp2 and if four sp3.
For hybridization to occur, it is necessary for the atom to satisfy the following conditions.
- Orbitals on a single atom would undergo hybridization.
- There should be very little difference of energy level between the orbitals mixing to form hybrid orbitals.
- Number of hybrid orbitals generated is equal to the number of hybridizing orbitals.
- The hybrid orbitals assume the direction of the dominating orbitals.
- It is the orbitals that undergo hybridization and not the electrons.
- The electrons waves in hybrid orbitals repel each other and thus tend to be farthest apart.