1 (1
or sin20 = -
2
- cos 20)
So, we can write Eq. A.l as
B U ~Icosaxdx = sin ax
Then,
(because, sin 2nn = 0)
,UNIT 3 ELECTRONIC THEORY OF
VALENCY -
structure
3.1 Introduction .
Objectives
3.2 Basic Concepts
Effective Nuclear Charge
Ionisation Energy
Electron Affinity
Electronegativity
3.3 Electrovalent or Ionic Bond
Properties of Ionic Compounds
lonic Rad~i
Lattice Energy
3.4 Covalent Bond
Characteristics of Covalent Molecules
Bond Enthalpy
Bond Lengths and Covalent Radii
3.5 Bond Polarity
3.6 Valence Shell Electron Pair Repulsion (VSEPR) Theory
3.7 Prediction of Shapes of Molecules and Ions Using VSEPR Theory
3.8 Summary
3.9 Terminal Questions
3.10 Answers .
3.1 INTRODUCTION
In Units 1 and 2, some concepts regarding atomic structure have been discussed in
detail. These ideas naturally lead you to think as to why, the atoms of only a few
elements combine among themselves or with those of others, to form stable
structures, called molecules. Hydrogen gas, for example, consists of H2 molecules,
each of which is made up of two atoms of hydrogen. In contrast to this, the He2
molecule is not formed by the combination of two atoms of helium. You may like to
know the nature of attractive forces, known as chemical bonds, that are responsible '
for the stability of some structures. Such an analysis can also help you in correlating
the physical and chemical characteristics of molecules to the type of bonding available
in them.
Many attempts were made in the past to explain the formation of stable molecules. On
the basis of electrolysis experiments, Berzelius (1812) concluded that some elements ,
have a positive electric charge and some a negative charge and that the attraction
between these opposite charges holds the elements together. This dualistic theory was
proposed before the discovery of electrons and protons. But Cannizzaro (1860)
showed that the elements like oxygen, hydrogen etc., are diatomic. This fact could not
be explained by the dualistic theory of Benelius. In 1852, Frankland proposed the
theory that elements have a definite capacity for combining to which the name
valence, was given later. It was for Kossel and Lewis (1916) to put forward theories of
electrovalent and covalent cdmpounds, respectively.
In this unit, we shall study the theories of Kossel and Lewis. The properties of ionic
compounds, the importance of the ratio of ionic radii in deciding the crystal geometry
and the significance of lattice energy are also discussed. We shall explain some of the
characteristics associated with the covalent molecules, such as bond enthalpy and
covalent radii. Finally we shall discuss, the relationship between the number of
electron pairs around the central atom and the shape of the molecules.
Qbjectives
After studying this unit, you should be able to :
define the terms such as effective nuclear charge, ionisation energy, electron
affinity, electronegativity and utilise them in predicting bond character,
, '0 define ionic bond and list the properties of ionic compounds,
explain the role of ionic radii in deciding crystal geometry,
calculate lattice energy and explain its importance in deciding stoichiometry,
stability and solubility of a crystal,
define covalent bond and compare the properties of covalent compounds with those
of ionic co..~nounds,
correlate bond e..rhalpy and bond stability,
explain the importance of covalent radii in estimating bond length, and
explain the shapes of molecules and ions based on VSEPR theory.
3:2 BASIC CONCEPTS
Atoms can combine with each other in different ways to form a large variety of
molecules. Really speaking the description of chemical bonds between atoms in a
molecule is essentially the description of electron distribution around nuclei of the
atoms in a molecule. There are three main types of bonding in molecules. One of them
is ionic bonding and it arises when one or more valence electrons are transferred from
one atom to another, resulting in the formation of positively and negatively charged
ions.
The second type of arrangement that holds atoms together in a molecule is known as
covalent bond. In this case, the b,ond results from the mutual sharing of electrons
between atoms of the same or different elements. The formation of Hzmolecule is a
case of covalent bond formation. Third type of bonding is coordinate bond and it
arises when both the shared electrons are provided by one atom only, known as the
,donor. A coordinate bond differs from the covalent bond only in the mode of
h a t i o n . Once a coordinate bond is formed, there is no way to distinguish it from
the covalent bond. The salient features of the three types of bonds described above are
listed in Table 3.1.
Table 3.1 :Bondlng in Molecules
Bo-tyPO Type delectmnk interaction b p k s
Electrovalent or Transfer of electron(s) from one atom to another Na+CI-,
ionic bond K+CI-, Cs+CI-
Covalent bond Equal contributionand sharingof electronsbetween F2,CH,,
H2,
atoms
Coordinate bond Contribution of electronsby one atom and sharing (cH3)3No, NH;
- by both the bonding atoms
One would like to know the factors on which the tendency of an atom to transfer or
share age or more of its electrons depends. There are various factors such as effective
nuclear charge, ionisation energy, electron affinity and electronegativity which
determine the above tendency. We shall explain the salient features of these factors
before going into the details of chemical bonding.
3.2.1 Effective Nuclear Chaige
The hold the nucleus has over an electron in a particular level, is decreased by other
electrons in the same and lower levels. In an atom with atomic number Z, the effect
of nuclear charge is not felt uniformly by the electrons in the various orbitals. The
extent to which other electrons decrease the nuclear charge over a particular electron
is given by screening constant, S. Hence the effective nuclear charge, Z*, is given by
the equation,
Z * = Z - S . ..(3-1) The outermost'she11in an atom is
called the valence shell. The
It is possible to calculate Z* from the values of Z and S. It is observed that there is a elearonr in this shell are called
steady increase in Z* across the elements in a period. The values of effective nuclear ,re, elearons.
charge felt by electrons in the valence shell of some elements are given in Table 3.2.
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