Hybridization

It is a phenomenon of intermixing the orbitals with comparable energy of an atom to give entirely new orbitals equal in number to the mixing orbitals having an identical shape and the same energy content.

Types of hybridization

1. SP³ hybridization

The phenomenon of intermixing one s-orbital and three p-orbital with comparable energy of an atom to give hybrid orbitals having identical shape and same energy content is called sp³ hybridization.
Each hybrid orbital has 25% s-character and 75% p-character. To minimise the repulsion between them, they are directed toward four corners of regular tetrahedral. So the shape of a molecule whose central atom undergoes sp³ hybridization is tetrahedral.

2. SP² hybridization

The phenomenon of intermixing one s-orbital and two p-orbital with comparable energy of an atom to give three hybrid orbitals having identical shapes and the same energy content is called sp2 hybridization.
Each sp² hybrid orbital has 33.3% s-character and 66.6% p-character. To minimise the repulsion, they are directed towards three corners of an equilateral triangle. So the shape of the molecule whose central atom undergoes sp² hybridization is trigonal planar.

3. SP hybridization

The phenomenon of intermixing one s-orbital and one p-orbital with comparable energy of an atom to give two hybrid orbitals having an identical shape and the same energy content is called sp hybridization.
Each sp hybrid orbital has its 50% s-character and 50% p-character. To minimise the repulsion, they are arranged in a linear manner. So the shape of the molecule whose central atom undergoes sp hybridization is linear.

Examples

1. CH₄ molecule

To form methane molecules, the orbitals of the carbon atom undergo sp³ hybridization to form four hybrid orbitals with identical shapes and the same energy content. They form four σ-bond with 1s orbital of the hydrogen atom. Since there is sp³ hybridisation, the shape of the molecule is tetrahedral.

2. C₂H₄ molecule

To form an ethane molecule, orbitals of carbon atoms undergo sp2 hybridization to form three hybrid orbitals having identical shapes and the same energy content. They form 2 σ-bond with 1s orbitals of the hydrogen atom and 1 σ-bond with a hybrid orbital of another carbon atom. The pure p-orbital of each carbon atom undergoes sidewise overlapping to form a π-bond. Since there is sp² hybridization, the shape of the molecule is trigonal planar.

3. C₂H₂ molecule

To form an acetylene molecule, orbitals of carbon atoms undergo sp hybridization to form two hybrid orbitals having an identical shape and the same energy content. They form 1 σ-bond with 1s orbitals of the hydrogen atom and 1 σ-bond with a hybrid orbital of another carbon atom. The pure p-orbital of each carbon atom undergoes sidewise overlapping to form two π-bond. Since there is sp hybridization, the shape of the molecule is linear.

4. NH₃ molecule

To form an ammonia molecule, orbitals of the nitrogen atom undergo sp3 hybridization to form four hybrid orbitals having an identical shape and the same energy content. They form 3 σ-bond with 1s orbitals of the hydrogen atom. The remaining one hybrid orbital is occupied by a lone pair of electrons. Since there is sp3 hybridization, the expected shape of the molecule is tetrahedral but the shape of the molecule is pyramidal. It is due to the presence of one lone pair of electrons which exerts greater repulsion with the bond pair due to which distortion in shape takes place.

Tricks to find the types of hybridization

Hybridization is determined by the sigma bond plus the lone pair of electrons. If:
sigma bond + lone pair = 2, it is sp hybridization
sigma bond + lone pair = 3, it is sp² hybridization
sigma bond + lone pair = 4, it is sp³ hybridization.

Q. Write the hybridization, shape and bond angle of the following compounds:

CH₄, BeCl₂, BCl₃, C₂H₄, C₂H₂, NH₃, PH₃, H₂O, H₂S

Dipole moment

It is defined as the product of the magnitude of charge developed in an atom of a molecule and the distance between the combining atoms. It is denoted by μ i.e. μ = q d. It is a vector quantity. Its value depends upon the geometry of the molecule. it is the resultant of the dipole moment of all the bonds involved in the molecule. Its unit is Debye (D).
If the dipole moment of a molecule is zero, the molecule is said to be non-polar. If the dipole moment of a molecule is greater than zero, the molecule is said to be polar.

Dipole moment and molecular structure

1. In completely symmetrical molecules (BCl₃, CO₂, CCl₄, CH₄, etc), the same bonds are distributed at equal angles. therefore the resultant dipole moment of bonds is zero. Such molecules are non-polar molecules.
   Example: In a CO₂ molecule, there is a 2 C=O bond. Each bond is polar. Since one C=O bond cancels the other, the net dipole moment of of molecule is zero. Hence CO₂ is a linear molecule. (O = C = O).

2. In a non-symmetrical molecule (H₂O, NH₃, CH₃Cl, HCl, etc), the resultant dipole moment of a molecule cannot be zero. Such molecules are polar molecules.
   Example: Water has a dipole of 1.84D. There are two O-H bonds and two lone pairs of electrons in water. Due to the lone pair of electrons, the resultant dipole moment cannot be zero. So water is a polar molecule with bent or angular or V geometry.

Applications of dipole moment

1. To predict the polar or nonpolar molecules
2. To predict the shape of the molecule
3. Gives an idea about the degree of polarity or ionic character of molecules.

Hydrogen bond

The force of attraction between hydrogen-bonded with a highly electronegative element (F or O or N) and a highly electronegative element (F or O or N) of the same or different molecule is called a hydrogen bond. It is not a covalent bond, so it is represented by the dotted line. eg. H-F……H-F…..H-F

Types of hydrogen bond

1. Intermolecular hydrogen bond

The force of attraction between hydrogen-bonded with a highly electronegative element (F or O or N) and highly electronegative element (F or O or N) of the different molecule is called an intermolecular hydrogen bond. examples:

Intramolecular hydrogen bond

The force of attraction between hydrogen-bonded with a highly electronegative element (F or O or N) and a highly electronegative element (F or O or N) of the same molecule is called an intramolecular hydrogen bond. examples:

Important points

1. The hydrogen atom is linked to an atom of highly electronegative. The atom of a highly electronegative element should be small in size.
2. The strength of the hydrogen bond increases with an increase in the electronegativity of the electronegative element.
3. It is responsible for an increase in the melting and boiling points of various compounds.
4. Polar compounds are soluble in water due to hydrogen bonds.

Van der Waal’s force

It is the short-lived attractive force between all kinds of atoms, molecules and ions when they come closer. It is an instantaneous force of attraction. it is weaker than a hydrogen bond. It increases with an increasing surface area of the molecule and increasing polarity in the molecule.

Types of Van der Waal’s force

1. Dipole-dipole interaction: It exists among polar molecules like HCl, HBr, H₂O, etc.
2. Ion-dipole interaction (Debye force): It exists among ion and polar compounds. eg, a force of attraction between NaCl and H₂O.
3. Instantaneous dipole-induced dipole interaction: It occurs between non-polar molecules like Ar, He, Cl₂, CH₄, etc.

Metallic bond

The force of attraction among the particles of metal is called a metallic bond. According to the electron gas model, it is the force of attraction between the mobile electron and positively charged Kernal (a portion of the atom left after the loss of valence electron).

Some Important Questions

1. Define hybridization. Mention the type of hybridization of the underlined atom.
   a. CH₄ b. C₂H₄ c. C₂H₂ d. BF₃ e. BeCl₂ f. NH₃ g. H₂O h. CO₂
2. Draw the orbital picture of acetylene and ammonia showing the sigma and pi bond.
3. Compare the bond angle of:
   a. NH₃ and BF₃
   b. H₂O and H₂S
   c. NH₃ and PH₃
4. How would you confirm that all four C-H bonds in methane are the same?
5. Draw the shape of sp, sp², and sp³ hybrid orbitals.
6. Define dipole moment. How does this concept help to predict the shape of H₂O and CO₂?
7. What is a hydrogen bond? Write its two applications.
8. What is an intramolecular hydrogen bond? Give an example of it.
9. Why is benzene molecular non-polar whereas chlorobenzene is non-polar?
10. H₂O exists as a liquid whereas H₂S exists as a gas, Why?
11. Ammonia is highly soluble in water, Why?
12. Write any two applications of dipole moment.
13. CO₂ is a linear molecule but H₂O is not, why?