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100 CHEMISTRYUNIT 4
CHEMICAL BONDING AND
MOLECULAR STRUCTURE
Scientists are constantly discovering new compounds, orderly
arranging the facts about them, trying to explain with the
existing knowledge, organising to modify the earlier views or
After studying this Unit, you will be evolve theories for explaining the newly observed facts.
able to
• understand K Ö ssel-Lewis
approach to chemical bonding;
• explain the octet rule and its Matter is made up of one or different type of elements.
limitations, draw Lewis Under normal conditions no other element exists as an
structures of simple molecules;
independent atom in nature, except noble gases. However,
• explain the formation of different a group of atoms is found to exist together as one species
types of bonds; having characteristic properties. Such a group of atoms is
• describe the VSEPR theory and called a molecule. Obviously there must be some force
predict the geometry of simple which holds these constituent atoms together in the
molecules; molecules. The attractive force which holds various
• explain the valence bond constituents (atoms, ions, etc.) together in different
approach for the formation of chemical species is called a chemical bond. Since the
covalent bonds; formation of chemical compounds takes place as a result
of combination of atoms of various elements in different
• predict the directional properties
of covalent bonds; ways, it raises many questions. Why do atoms combine?
Why are only certain combinations possible? Why do some
• explain the different types of atoms combine while certain others do not? Why do
hybridisation involving s, p and
d orbitals and draw shapes of molecules possess definite shapes? To answer such
simple covalent molecules; questions different theories and concepts have been put
forward from time to time. These are Kössel-Lewis
• describe the molecular orbital
theory of homonuclear diatomic approach, Valence Shell Electron Pair Repulsion (VSEPR)
molecules; Theory, Valence Bond (VB) Theory and Molecular Orbital
(MO) Theory. The evolution of various theories of valence
• explain the concept of hydrogen and the interpretation of the nature of chemical bonds have
bond.
closely been related to the developments in the
understanding of the structure of atom, the electronic
configuration of elements and the periodic table. Every
system tends to be more stable and bonding is nature’s
way of lowering the energy of the system to attain stability.
2022-23
,CHEMICAL BONDING AND MOLECULAR STRUCTURE 101
4.1 KÖSSEL-LEWIS APPROACH TO the number of valence electrons. This number
CHEMICAL BONDING of valence electrons helps to calculate the
In order to explain the formation of chemical common or group valence of the element. The
bond in terms of electrons, a number of group valence of the elements is generally
attempts were made, but it was only in 1916 either equal to the number of dots in Lewis
when Kössel and Lewis succeeded symbols or 8 minus the number of dots or
independently in giving a satisfactory valence electrons.
explanation. They were the first to provide Kössel, in relation to chemical bonding,
some logical explanation of valence which was drew attention to the following facts:
based on the inertness of noble gases.
• In the periodic table, the highly
Lewis pictured the atom in terms of a electronegative halogens and the highly
positively charged ‘Kernel’ (the nucleus plus electropositive alkali metals are separated
the inner electrons) and the outer shell that by the noble gases;
could accommodate a maximum of eight
electrons. He, further assumed that these • The formation of a negative ion from a
halogen atom and a positive ion from an
eight electrons occupy the corners of a cube
alkali metal atom is associated with the
which surround the ‘Kernel’. Thus the single
gain and loss of an electron by the
outer shell electron of sodium would occupy
respective atoms;
one corner of the cube, while in the case of a
noble gas all the eight corners would be • The negative and positive ions thus
occupied. This octet of electrons, represents formed attain stable noble gas electronic
a particularly stable electronic arrangement. configurations. The noble gases (with the
Lewis postulated that atoms achieve the exception of helium which has a duplet
stable octet when they are linked by of electrons) have a particularly stable
chemical bonds. In the case of sodium and outer shell configuration of eight (octet)
chlorine, this can happen by the transfer of electrons, ns2np6.
an electron from sodium to chlorine thereby
–
• The negative and positive ions are
giving the Na+ and Cl ions. In the case of stabilized by electrostatic attraction.
other molecules like Cl2, H2, F2, etc., the bond For example, the formation of NaCl from
is formed by the sharing of a pair of electrons sodium and chlorine, according to the above
between the atoms. In the process each atom scheme, can be explained as:
attains a stable outer octet of electrons. Na → Na+ + e
–
Lewis Symbols: In the for mation of a [Ne] 3s1 [Ne]
molecule, only the outer shell electrons take –
Cl + e → Cl–
part in chemical combination and they are
known as valence electrons. The inner shell [Ne] 3s2 3p5 [Ne] 3s2 3p6 or [Ar]
–
electrons are well protected and are generally Na+ + Cl– → NaCl or Na+Cl
not involved in the combination process. Similarly the formation of CaF2 may be
G.N. Lewis, an American chemist introduced shown as:
simple notations to represent valence –
Ca → Ca2+ + 2e
electrons in an atom. These notations are
called Lewis symbols. For example, the Lewis [Ar]4s2 [Ar]
– –
symbols for the elements of second period are F +e → F
as under: 2
[He] 2s 2p 5 [He] 2s2 2p6 or [Ne]
– –
Ca2+ + 2F → CaF2 or Ca2+(F )2
The bond formed, as a result of the
Significance of Lewis Symbols : The electrostatic attraction between the
number of dots around the symbol represents positive and negative ions was termed as
2022-23
,102 CHEMISTRY
the electrovalent bond. The electrovalence chlorine atoms attain the outer shell octet of
is thus equal to the number of unit the nearest noble gas (i.e., argon).
charge(s) on the ion. Thus, calcium is The dots represent electrons. Such
assigned a positive electrovalence of two, structures are referred to as Lewis dot
while chlorine a negative electrovalence of structures.
one.
The Lewis dot structures can be written
Kössel’s postulations provide the basis for for other molecules also, in which the
the modern concepts regarding ion-formation combining atoms may be identical or
by electron transfer and the formation of ionic different. The important conditions being that:
crystalline compounds. His views have proved
• Each bond is formed as a result of sharing
to be of great value in the understanding and
of an electron pair between the atoms.
systematisation of the ionic compounds. At
the same time he did recognise the fact that • Each combining atom contributes at least
a large number of compounds did not fit into one electron to the shared pair.
these concepts. • The combining atoms attain the outer-
shell noble gas configurations as a result
4.1.1 Octet Rule
of the sharing of electrons.
Kössel and Lewis in 1916 developed an
• Thus in water and carbon tetrachloride
important theory of chemical combination
molecules, formation of covalent bonds
between atoms known as electronic theory
can be represented as:
of chemical bonding. According to this,
atoms can combine either by transfer of
valence electrons from one atom to another
(gaining or losing) or by sharing of valence
electrons in order to have an octet in their
valence shells. This is known as octet rule.
4.1.2 Covalent Bond
Langmuir (1919) refined the Lewis
postulations by abandoning the idea of the
stationary cubical arrangement of the octet, Thus, when two atoms share one
and by introducing the term covalent bond. electron pair they are said to be joined by
The Lewis-Langmuir theory can be a single covalent bond. In many compounds
understood by considering the formation of we have multiple bonds between atoms. The
the chlorine molecule,Cl2. The Cl atom with formation of multiple bonds envisages
electronic configuration, [Ne]3s 2 3p5, is one sharing of more than one electron pair
electron short of the argon configuration. between two atoms. If two atoms share two
The formation of the Cl2 molecule can be pairs of electrons, the covalent bond
understood in terms of the sharing of a pair between them is called a double bond. For
of electrons between the two chlorine atoms, example, in the carbon dioxide molecule, we
each chlorine atom contributing one electron have two double bonds between the carbon
to the shared pair. In the process both and oxygen atoms. Similarly in ethene
molecule the two carbon atoms are joined by
a double bond.
or Cl – Cl
Covalent bond between two Cl atoms Double bonds in CO2 molecule
2022-23
, CHEMICAL BONDING AND MOLECULAR STRUCTURE 103
in subtraction of one electron from the total
number of valence electrons. For example,
2–
for the CO3 ion, the two negative charges
indicate that there are two additional
electrons than those provided by the
+
neutral atoms. For NH 4 ion, one positive
charge indicates the loss of one electron
C2H4 molecule from the group of neutral atoms.
• Knowing the chemical symbols of the
When combining atoms share three
combining atoms and having knowledge
electron pairs as in the case of two
of the skeletal structure of the compound
nitrogen atoms in the N2 molecule and the
(known or guessed intelligently), it is easy
two carbon atoms in the ethyne molecule,
to distribute the total number of electrons
a triple bond is formed.
as bonding shared pairs between the
atoms in proportion to the total bonds.
• In general the least electronegative atom
occupies the central position in the
molecule/ion. For example in the NF3 and
2–
N2 molecule CO3 , nitrogen and carbon are the central
atoms whereas fluorine and oxygen
occupy the terminal positions.
• After accounting for the shared pairs of
electrons for single bonds, the remaining
electron pairs are either utilized for multiple
C2H2 molecule
bonding or remain as the lone pairs. The
4.1.3 Lewis Representation of Simple basic requirement being that each bonded
Molecules (the Lewis Structures) atom gets an octet of electrons.
The Lewis dot structures provide a picture Lewis representations of a few molecules/
of bonding in molecules and ions in terms ions are given in Table 4.1.
of the shared pairs of electrons and the
octet rule. While such a picture may not Table 4.1 The Lewis Representation of Some
explain the bonding and behaviour of a Molecules
molecule completely, it does help in
understanding the formation and properties
of a molecule to a large extent. Writing of
Lewis dot structures of molecules is,
therefor e, very useful. The Lewis dot
structures can be written by adopting the
following steps:
• The total number of electrons required for
writing the structures are obtained by
adding the valence electrons of the
combining atoms. For example, in the CH4
molecule there are eight valence electrons
available for bonding (4 from carbon and
4 from the four hydrogen atoms).
• For anions, each negative charge would
mean addition of one electron. For * Each H atom attains the configuration of helium (a duplet
cations, each positive charge would result of electrons)
2022-23
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