Chemical Bond Notes
1/20 or 1/21 thru 1/22 to 1/23
Introduction
Atoms of all the elements except noble gases have incomplete outermost orbits and tends to complete them by chemical combination with the other atoms.
In 1916, W Kossel described the ionic bond which is formed by the transfer of electron from one atom to another and also in 1916 G.N Lewis described about the formation of covalent bond which is formed by the mutual sharing of electrons between two atoms.
Both these scientists based their ideas on the fact that atoms greatest stability when they acquire a noble gas/inert gas electronic configuration.
Definition
When two or more than two atoms are combined with each other in order to complete their octet a link between them is produced which is known as chemical bond.
OR
The force of attraction which holds atoms together in the molecule of a compound is called chemical bond.
Types of Chemical Bonds
There are three main types of chemical bond.
1. Ionic bond
2. Covalent bond
Ionic Bond
A chemical bond which is formed by the complete shifting of electron between two atoms is called ionic bond
OR
The electrostatic attraction between positive and negative ions is called ionic bond.
Conditions for the Ionic Bond Formation
1. Electro negativity
Ionic bond is formed between the element having a difference of electro negativity more than 1.7 or equal to 1.7 eV.
Therefore ionic bond is generally formed between metals (low electronegative) and non-metal (high electronegative) elements.
2. Ionization Potential
Ionic bonds are formed by the transference of electron from one atom to another, so in the formation of ionic bonds an element is required which can lose its electrons from the outer most shell. It is possible to remove electrons from the outermost shell of metals because of their low ionization potential values.
3. Electron Affinity
In the formation of an ionic bond an element is also required which can gain an electron, since non-metals can attract electrons with a greater force due to high electro negativity. So a non-metal is also involved in the formation of ionic bond due to high electron affinity.
Example of Ionic Bond:
In order to understand ionic bond consider the example of NaCl. During the formation of Ionic bond between
Na and Cl
2
, Sodium loses one electron to form Na
+
ion while chlorine atom gains this electron to form Cl
-
ion.
When Na
+
ion and Cl
-
ion attract to each other NaCl is formed. The stability of NaCl is due to the decrease in the energy. These energy changes which are involved in the formation of ionic bond between Na and Cl are as follows.

i.
Sodium has one valence electron. In order to complete its octet Na loses its valence electron. The loss of the valence electron required 495 kJ/mole. Energy absorption
Na —-> Na+ + e- ………………….. ΔH = 495 kJ/mole ii.
Chlorine atom has seven electrons in its valence shell. It requires only one electron to complete its octet, so chlorine gains this electron of sodium and release s 348 kJ/mole energy. Energy release
Cl + e-

Cl- …………………. ΔH = -348 kJ/mole
Here the energy difference is 147 kJ/mole (495 – 348 = 147). This loss of energy is balanced when oppositely charged ions are associated to form a crystal lattice.
iii . In third step, a positively charged Na
+
ion and negatively charged Cl
-
ion attract to each other and a crystal lattice is formed with a definite pattern.
Na
+
(g) + Cl
-
(g)

Na+Cl ……….. ΔH = – 788 kJ/mole
This energy which is released when one mole of gaseous ions arrange themselves in definite pattern to form a lattice is called lattice energy.
From this example, we can conclude that it is essential from the formation of ionic bond that the sum of energies released in the second and third steps must be greater than the energy required for the first step.
Characteristics of Ionic Compounds
1.
In an ionic compound, the oppositely charged ions are tightly packed with each other, so these compounds exist in solid state.
2.
Due to strong attractive forces between ions the larger amount of energy required to melt or to boil the compound and hence the melting and boiling point of the ionic compound are generally high.
3.
Ionic compounds are soluble in water but insoluble in organic solvents like benzene, CC l4
, etc.
4.
In the aqueous solution, the ionic compounds are good electrolytes, because in water the interionic forces are so weakened that the ions are separated and free to move under the influence of electric current. Due to this free movement of ions, the ionic compounds conduct electricity in their solutions.
Covalent Bond
A link which is formed by the mutual sharing of electrons between two atoms is called covalent bond.
In the formation of covalent bond, mutual sharing of electron takes place. This mutual sharing is possible in non-metals, therefore covalent bonds generally form between the atoms of non-metals.
In Cl
2
molecule, two atoms of chlorine are combined with each other to form Cl
2
molecule. Each atom of chlorine has seven electrons in its valence shell. These atoms are united with each other by sharing one of its valence electrons.
Cl + Cl

:Cl :Cl or Cl – Cl
In this molecule, one shared pair of electrons forms a single covalent bond between two chlorine atoms. With the formation of a covalent bond the energy of the system is also decreased.
Cl + Cl  Cl – Cl ………….. ΔH = – 242 kJ / mole
The released energy lowered the energy of the molecule and the stability of the compound is also increased.
Types of Covalent Bond
There are three main types of covalent bond.
1. Single Covalent Bond
When a covalent bond is formed by sharing of one electron from each atom, that it is called single covalent bond and denoted by (-) single line between the two bonded atoms e.g.
Cl – Cl, H – H, H – Br etc. includes the seven diatomic molecules (H,N,O,F,Cl,Br,I)

2. Double Covalent Bond
In a covalent bond, if two electrons are shared from each of the bonded atoms then this covalent bond is called double covalent bond and denoted by (=) two lines e.g.
O = O, O : : O
3. Triple Covalent Bond
When a covalent bond is formed by sharing of three electrons from each atom then this type of covalent bond is called triple covalent bond, and denoted by (≡) three lines between the two bonded atoms e.g.
N : : N :, N ≡ N
The bond distance of multiple bonds are shorter and the bond energies are higher.
Characteristics of Covalent Compounds
The main characteristics properties of covalent compounds are as follows
1.
The covalent compounds exist as separate covalent molecules, because the particles are electrically neutral so they exist in solid, liquid or gaseous states. There is an intermolecular force of attraction among the molecules.
2.
Since the covalent compound exist in all the three states of matter their melting points and boiling point may be high or low.
3.
Covalent compounds are non-electrolytes so they do not conduct electricity from their aqueous solution.
4.
Covalent compounds are generally insoluble in water and similar polar solvent but soluble in the organic solvents.
Bond Energy
The amount of energy required to break a bond between two atoms in a diatomic molecule is known OR
The energy released in forming a bond from the free atoms is also known as Bond Energy. i.
The bond energy for hydrogen molecule is
H – H(g)  2 H(g) …………………….. ΔH = 435 kJ/mole
OR
H(g) + H(g)

H – H ………………….. ΔH = 435 kJ/mole
It can be observed from this example that the breaking of bond is endothermic whereas the formation of the bond is exothermic.
ii. The bond energy for oxygen molecule is
O = O(g)

2 O(g) …………………… ΔH = 498 kJ/mole
OR
O(g) + O(g)

O = O ……………….. ΔH = -498 kJ/mole
Bond energy of a molecule also measure the strength of the bond. Generally bond energies of polar bond are greater than pure covalent bond.
Example
Cl – Cl

2 Cl …………………… ΔH = 244 kJ/mole
H – Cl

H
+
+ Cl
-
………………. ΔH = 431 kJ/mole
The value of bond energy e.g., triple bonds are usually shorter than the double bond therefore the bond energy for triple bond is greater than double bond.
Valence Shell Electron Pair Repulsion Theory
The covalent bonds are directed in space to give definite shapes to the molecules. The electrons pairs forming the bonds are distributed in space around the central atom along definite directions. The shared electron pairs as well as the lone pair of electrons are responsible for the shape of molecules.
The known geometries of many molecules based upon measurement of bond angles shows that lone pairs of electrons occupy more space than bonding pairs. The repulsion between electronic pairs in the valence shell decreases in the following order.
Lone Pair – Lone Pair > Lone Pair – Bond Pair > Bond Pair – Bond Pair