CHEM1101: CHEMISTRY (EEE/CSE/IPE) CHAPTER 5

CHEMICAL BONDS

A chemical bond is defined as a strong attractive force that exists between two or more atoms
to hold them together as a stable molecule in a substance.

Some definitions to understand chemical bonding:

Valence electron: The electrons in the outer most energy level in an atom that takes part in
chemical bonding are called valence electrons.
Bonding electrons: The valence electrons actually involved in bond formation are called
bonding electrons.
Octet rule: In chemical bond formation, atoms interact- (i) by losing, (ii) by gaining or (iii) by
sharing electrons to acquire a stable noble gas configuration. The tendency for atoms to have
eight electrons in the outer shell is known as Octet rule.

Main types of bond:

(1) Ionic or electrovalent bond, (2) Covalent bond, (3) Coordinate covalent bond

Other important bonds:

(1) Metallic bond, (2) Hydrogen bond

IONIC BOND
An ionic bond is a chemical bond formed by the electrostatic attraction between positive and
negative ions. The bond forms between two atoms when one or more electrons are transferred
from the valence shell of one atom to the valence shell of the other. The atom that loses
electrons becomes a cation (positive ion), and the atom that gains electrons becomes an anion
(negative ion).
.. _
A . B: A+ B
..

1 7 (cation) (anion)
valence valence
electron electron

Conditions for formation of ionic bond:

(1) Bond formations occur between the metal and nonmetal. Metal is donor and nonmetal
is acceptor.

(2) Net lowering of energy:

To form a stable ionic compound, there must be a net lowering of the energy. For
example- formation of NaCl molecule
(a) Na ― e → Na+ ―119 kcal (energy requires)
(b) Cl + e → Cl‾ +85 kcal (energy releases)
(c) Na+ + Cl‾→ Na+ Cl‾ +187 kcal (energy releases)
Net energy releases, 187 + 85 ― 119 = 153 kcal
This causes lowering of energy of the process, hence ionic bond formed.

(3) Electro negativity difference between the atoms

Na → 0.9

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CHEM1101: CHEMISTRY (EEE/CSE/IPE) CHAPTER 5

Cl → 3.0  Difference, 3.0 ― 0.9 = 2.1

Hence ionic bond will form.

Factors that govern the formation of ionic bond:

(1) Ionization energy,
(2) Electron affinity,
(3) Lattice energy (The energy of an ionic solid is a measure of strength of bonds)

Some examples of ionic bond:

NaCl .. .. _ _
Na . Cl : Na+ . Cl
..
: or Na+ Cl
..

2, 8, 1 2, 8, 7 2, 8 2, 8, 8

CaO
.. .. _ _
Ca : Ca2 + O. : 2 Ca2+ O 2
.O. . or

2, 8, 8, 2 2, 6 2, 8, 8 2, 8
Characteristics of ionic compounds:
(1) solids at room temperature
(2) high melting and boiling points
(3) hard and brittle,
(4) soluble in water but insoluble in organic solvents,
(5) conductors of electricity,
(6) don’t exhibit isomerism
(7) undergo ionic reactions which are fast.

COVALENT BOND
The attractive force between atoms created by sharing of an electron-pair.
Covalent bond
.B A .
A B Or A B

shared electron pair

Conditions for formation of covalent bond:
(1) Bond formations occur between two same or different nonmetals.
(2) Equal electronegativity
(3) Equal sharing of electrons

Some examples of covalent compounds:

H2 .H
H H . H Or H H

H2 O H . . H . .
O H O H Or H O H

sharing
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CHEM1101: CHEMISTRY (EEE/CSE/IPE) CHAPTER 5

Examples of multiple covalent bonds:

O2 .. .. ..
O :O O :O or O = O:
.. ..
2, 6 2, 6 2, 8 2, 8

.. .. ..
CO2 :O: ..
C :O: :O: C :O: or O=C=O

Characteristics of covalent compounds:

(1) Gases, liquids or solids at room temperature,
(2) Low melting points and boiling points,
(3) Soft, much readily broken,
(4) Insoluble in water but soluble in organic solvents,
(5) Non-conductors of electricity,
(6) Exhibit isomerism,
(7) Undergo molecular reactions which are generally slow

COORDINATE COVALENT BOND

A covalent bond in which both electrons of the shared pair come from one of the two atoms
(or ions).
Examples:
H H H
+
+
NH4 H N: H+ H N H or H N H

H H H
ammonium ion

H3O+ .. ..
+
+
H O: H+ H O H or H O H

H H H
hydronium ion
O3 ..
: O O O
O=O .O.

METALLIC BONDING

The type of bonding which holds the atoms together in metal crystal. A metallic bonding is
the electrostatic force of attraction.
_ _ _ _ _
e e e e e
_ _ _ _ _
e e e e e
_ _ _ _ _
e e e e e

Figure: The electron sea model of metallic bonding

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CHEM1101: CHEMISTRY (EEE/CSE/IPE) CHAPTER 5

Electric conductivity: It is attended by displacement of electron as a result of voltage at both

ends. The mobile electrons are responsible for electric conductivity.
_ _ _
Entering e e e
electricity _ _ _ Exiting
e e e electricity
_ _ _
e e e
Figure: Electrical conductivity by flow of electrons
based on electron sea model

Heat conductivity: It is done by absorbing heat energy and increasing vibrational motion of
electron which collide with adjacent electron and transfer energy.

_ _ _
heat e e e
energy
_ _ _
e e e

Ductility (that can be made into thin wire) and Malleability (can be pressed into different
shapes without breaking): The phenomenon of restoring crystal lattice of metal after applying
force, for example by hammering. The sea of electrons adjust position rapidly and the internal
structure remains unchanged.
_ _ _ _ _ _
e e e e e e
_ _ _ _ _ _
Stress e e e e e
_ _
e
_ _ _ _
e e e e e e
Before After

HYDROGEN BOND

◼ When hydrogen (H) is covalently bonded to a highly electronegative atoms (X= O, F,

Cl), the shared electron pair is pulled so close to X that a strong dipole results.
◼ X δ- :─ H δ+ hydrogen bond

.. .. ..
H O: H O: H O:

H H H
Liquid water
BOND LENGTH, BOND ANGLE AND BOND ENERGY

The distance between the centers of two nuclei of atoms connected by a chemical bond is
known as ‘bond length’. It depends on number of bonds. When number of bonds increases,
bond length decreases due to more attraction.

The angle between the directions of two bonds in a molecule is called the ‘bond angle’. Bond
angle depend upon factors, (i) charge distribution, (ii) geometry of the molecules, (iii)
symmetry, (iv) hybridization etc.

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CHEM1101: CHEMISTRY (EEE/CSE/IPE) CHAPTER 5

Bond energy is defined as the energy required to split the molecule into atoms. It depends on
types of bonding- single, double or triple bonds.

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