The Science of Chemical Bonds

The Science of Chemical Bonds

Chemical Bonding

The force that holds together the different atoms in a molecule is called a chemical bond. There are many types of chemical bonds.

Ionic bond or (Electrovalent bond):
A bond formed by the complete transfer of one or more electrons from one atom to another atom is called an ionic bond. Example:

Formation of NaCl:
Na⁺ + Cl⁻ → Na⁺Cl⁻

Condition of ionic bond:

• Ionization energy of metal should be low

• Electron affinity of non-metal should be high.

Properties of ionic compounds:

• (a) Ionic compounds have high melting point & boiling point.

• (b) Ionic compounds are good conductors of electricity in molten state or in water.

• (c) Ionic compounds are bad conductors of electricity in solid state.

• (d) Ionic compounds are soluble in water.

• (e) Ionic compounds are insoluble in non-polar solvents like Benzene, Carbon tetrachloride etc.

Common salt (NaCl) is soluble in water. It means that in this case, hydration energy released by water is more than the lattice energy of sodium chloride (NaCl).

Covalent bond:

A bond formed between two same or different atoms by mutual contribution and sharing of electrons is called a covalent bond. Example:

H₂ molecule:
H - H → H - H

Cl₂ molecule:
Cl - Cl → Cl - Cl

Lone pair of electrons:
The pair of electrons which do not take part in covalent bond formation are called Lone pair electrons. For example:
There is one Lone pair in ammonia (NH₃) and two Lone pairs in water (H₂O).

In NH₃:
H - N : H
H

In H₂O:
H : O - H

Properties of covalent compounds:

• (a) Covalent compounds have high melting point & boiling point.

• (b) They are generally bad conductors of electricity (except graphite).

Properties of Covalent Compounds:

• (c) They are generally insoluble in water.

• (d) They are generally soluble in organic solvents like benzene, acetone, chloroform, etc.

• (e) Covalent bonds are directional.

Covalent bonds are of two types:

1. Non-polar covalent bonds

2. Polar covalent bonds

When a covalent bond is formed between two similar atoms of an element, it is called a non-polar covalent bond.

Example:
H - H, O = O, N ≡ N

When a covalent bond is formed between different atoms of the elements, it is called a polar covalent bond.

Example:
H - Cl, H - O, H - N

Co-ordinate bond (or Dative bond):

A co-ordinate bond is a special type of covalent bond in which one atom donates electrons to another atom. The bonding between donor to acceptor atom is called co-ordinate bond. It is denoted by:

Example:
O : → S : → O → S

Ammonium chloride (NH₄Cl) has all the three types of bonds: ionic, covalent, and co-ordinate bonds.

Sigma bond (σ-bond):

A bond formed by the linear overlapping of atomic orbitals is called sigma bond. Since, the extent of overlapping of atomic orbitals in σ-bond is large, hence σ-bond is a strong bond.

Pi bond (π-bond):

A bond formed by the sideways or lateral overlapping of atomic orbitals is called pi-bond. Since, in this case, extent of overlapping of atomic orbitals is lesser than σ-bond, hence π-bond is a weak bond.

Example:
H - H : O = O : N ≡ N

Hybridisation:

The phenomenon of mixing of two or more atomic orbitals of equivalent energies to form new type of identical number of orbitals is called hybridisation and the new type of orbitals obtained are called hybrid orbitals.

Types of hybridisation and geometry:

Bond energy:

The amount of energy required to break one mole bonds of a particular type between the atoms in the gaseous state of a substance is called bond energy. The bond energy depends upon the following factors:

1. Size of atom

2. Multiplicity of bonds

   • Greater the size of atoms, lesser will be bond energy.

   • Greater the bond multiplicity, more will be bond energy.

Bond energy:
Single bond < Double bond < Triple bond

Bond Length:

The average equilibrium distance between the centers of the two bonded atoms is called bond length. The bond length is influenced by the following factors:

1. Size of atoms

2. Multiplicity of bonds

• Greater the size of atoms, greater will be bond length.

• Greater the multiplicity of bonds, lesser will be bond length.

Hydrogen Bond:

When a hydrogen atom is present between two most electronegative atoms (N, O, F), and it is bonded to one by a covalent bond and to the other by a weak force of attraction, this is called a hydrogen bond. It is denoted by the symbol "....".
Example:

1. (HF)ₙ .... H - F .... H - F .... H - F

2. (H₂O)ₙ .... H - O .... H - O .... H - O .... H H H

3. H₂O is liquid due to formation of hydrogen bond. H₂S does not form hydrogen bond. So, it is gas at room temperature.

4. In water, each H₂O molecule is surrounded by four other H₂O molecules by hydrogen bond. It means that the maximum number of hydrogen bonds in a H₂O molecule is four. In water, each oxygen atom is surrounded by two H₂O molecules with hydrogen bonds.

Types of Hydrogen Bonding:

There are two types of hydrogen bonding:

1. Intermolecular hydrogen bond

2. Intramolecular hydrogen bond

Intermolecular hydrogen bond:
This arises when hydrogen bonding occurs between two or more molecules. In this case, the melting point (m.p.) and boiling point (b.p.) of the compound increases due to molecular association.
Example:
.... H - F .... H - F .... H - F .... H

Intramolecular hydrogen bond:
When hydrogen bonding occurs within a molecule, it is called intramolecular hydrogen bonding. Due to cyclisation, the m.p. and b.p. of the compound decreases in this case.
Example:
H - N (with hydrogen bond) - O

Due to intermolecular hydrogen bonding between alcohol and water, alcohol is soluble in water.

Example:
CH₃ - O - H .... H
Methyl alcohol and Water