Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

CHEMICAL BONDING AND
MOLECULAR STRUCTURE
Introduction:
Structure and Bonding is the heart of chemistry
...
The important aspect of each type of
force is its relative strength, how rapidly it decreases with increasing distance and
whether it is directional in nature or not
...
Nature loves stability and bond formation is associated with stability
...
Noble gas electronic configuration can be achieved by
1
...
Mutual sharing of electrons
3
...
On
the basis of this chemical bonds are following type:
1
...
Covalent bond
3
...
Metallic bond
5
...
van der Waal’s bond

Lewis Dot Structures
Valence Electrons: In the formation of a molecule only the outer shell electrons take part
in chemical bond combination and they are known as valence electrons
...


Octet Rule

It is proposed by Kossel and Lewis and according to this, "Every atom has a tendency to
attain Noble gas electronic configuration or to have 8 valence electrons"
...

According to Lewis, only those compounds will be stable which follow octet rule
...

It is expressed as:

Ionic Bond
An ionic bond is formed by complete transference of one or more electrons from the
valence shell of one atom to the valence shell of another atom
...
The atom which loses
electron becomes a positive ion and the atom which gains electron becomes negative
ion
...

Characteristics of Ionic Compounds:
1
...

2
...

3
...

4
...

5
...

6
...
They are bad conductors of electricity in solid state because ions are unable to move
...
If the combining atoms are same the covalent molecule is known as
homoatomic
...


Valence Bond Theory (VBT)
Valence bond theory was introduced by Heitler and London (1927) and developed by
Pauling and others
...
Let two hydrogen atoms A and B having their nuclei NA and
NB and electrons present in them are eA and eB
...

1
...

2
...

Attractive forces tend to bring the two atoms closer whereas repulsive forces tend to
push them apart
...
This partial interpenetration of atomic orbitals is called overlapping of
atomic orbitals
...


Types of overlapping
The covalent bonds can be classified into two different categories depending upon the
type of overlapping
...
The atomic orbitals overlap along the internuclear axis and
involve end to end or head on overlap
...
s-s

overlap: In this case, there is overlap of two half-filled s-orbitals along the
internuclear axis as shown below
...
s-p

overlapping: It involves the overlapping of half filled s-orbitals of one atom with the
half filled p-orbitals of the other atom
...

iii
...
The bond as formed is called p-p
sigma bond
...
The orbitals formed due to sidewise overlapping consists of two
saucer type charged clouds above and below the plane of the participating atoms
...


Hybridisation: Hybridisation is the process of intermixing of the orbitals of slightly
different energies so as to redistribute their energies, resulting in the formation of new set
of orbitals of equivalent energies and shape
...

Salient Features of Hybridisation:
i
...

ii
...

iii
...
iv
...

Important conditions for hybridisation:
i
...


ii
...

iii
...
It

of electron is not essential condition prior to hybridisation
...


Types of hybridisation
There are many different types of hybridisation depending upon the type of orbitals
involved in mixing such as sp3, sp2, sp, sp3d, sp3d2 etc
...


sp-hybridisation: In this hybridisation one s and one p orbitals hybridise to produce two
equivalent hybrid orbitals, known as sp hybrid orbitals
...
Each of hybrid orbitals has 50% s-character and 50%
p-character
...

ii
...
sp2 hybrid orbitals are larger in size than
sp-hybrid orbitals but slightly smaller than that of sp3 hybrid orbitals
...
33%) s-character and 2/3 (or 66
...
Example, BF3, BCl3, BH3 etc
...
sp

d-hybridisation: This type of hybridisation involves mixing of one s, three p and one d
orbitals to form five sp3d hybridised orbitals which adopt trigonal bipyramidal
...
Under the conditions of bond formation the 3s-electrons get unpaired and one of the
electron is promoted to vacant 3dz2 orbital
...
These six orbitals are directed towards the
corners of an octahedron and lie in space at an angle of 90° to one another
...
In the excited state the
electron pairs in 3s and 3px orbitals get unpaired and one out of each pair is
promoted to vacant 3dz2 and 3dx2-y2 orbitals
...
It was further developed by Nyholm and
Gillespie (1957)
...
The

exact shape of molecule depends upon the number of electron pairs (bonded or
non bonded) around the central atoms
...
The

electron pairs have a tendency to repel each other since they exist around the
central atom and the electron clouds are negatively charged
...
Electron

pairs try to take such position which can minimize the rupulsion between

them
...
The

valence shell is taken as a sphere with the electron pairs placed at maximum

distance
...
A

multiple bond is treated as if it is a single electron pair and the electron pairs which
constitute the bond as single pairs
...
It is defined as the angle between the orbitals containing bonding
electron pairs around the central atom in a molecule/complex ion
...

ii
...

iii
...
For example, the lattice enthalpy of NaCl is 788 kJ mol–1
...


iv
...
The bond order may
be a whole number, a fraction or even zero
...


Bond Enthalpy: It is defined as the amount of energy required to break one mole of
bonds of a particular type between two atoms in a gaseous state
...
For example, the H – H bond enthalpy in hydrogen molecule is 435
...

v
...
Hund and R
...
Mulliken in 1932
...
All the electrons belonging to the atoms constituting a molecule are considered
to be moving along the entire molecule under the influence of all the nuclei
...
These orbitals are called molecular orbitals
...
This phenomenon is known as photo electric effect
...
These structures are known as resonating
structures or canonical structures
...
This phenomenon is known as resonance
...
Thus, partial positive charge develops on hydrogen atom which forms a
bond with the other electronegative atom
...


Types of Hydrogen Bonding:
i
...
Example : H – F, HF and water, NH3, NH3 and water, alcohol,
alcohol and water etc
...
Intramolecular

hydrogen bonding: It is a type of hydrogen bonding within the molecule
...


Applications of Hydrogen Bonding
i
...
For example, H2O is liquid at
room temperature whereas H2S is gas
...

ii
...

iii
...
For
example, NH3 has higher boiling point than PH3
...

iv
...
In this structure, some vacant spaces are formed, which decrease the
density of ice
...

This model could easily explain the following properties of metals:
i
...
High thermal conductivity
iii
...
Malleability
v
...
Tensile strength
vii
...
Chemical Bond: The force of attraction which holds various chemical entities in
different species
...
Electrovalent Bond : The attractive force between the oppositely charged ions which
comes into existence by the transference of electrons
...
Electrovalency: The number of electrons which an atom loses or gains while forming
ionic or electrovalent bond
...
Covalent Bond: The bond comes into existence by the mutual sharing of electrons by
the atoms participating in bonding
...
Valence Bond Approach of Covalent Bond: The bond is formed by the overlapping of
halffilled atomic orbitals having electrons with opposite spins
...
Covalency: The number of half-filled atomic orbitals which an atom provides for
participation in overlapping at the time of bonding
...
Dative Bond or Co-ordinate Bond: The bond is formed by sharing of electrons in which
the shared pair of electrons is contributed by one of the atom called donor while the
other atom is called acceptor
...
Hybridisation: The process of mixing or merging of orbitals (of slightly different
energies) of an atom to form another set of orbitals with equivalent shape and
energy
...
Geometry of the Molecule: The definite relative arrangement of the bonded atoms in a
molecule
...
Regular and Irregular Geometry: The molecule is said to possess regular geometry if
the repulsive interactions among the electron pair around the central atom are of
equal magnitude
...

11
...

12
...

13
...

14
...

15
...

16
...
The various electronic
arrangements are called resonating structures or canonical structure
...
Molecular Orbital Theory (MOT): According to this theory, in molecules the electrons
are present in new orbitals called molecular orbitals
...

18
...


CHEMISTRY CHEMICAL BONDING AND MOLECULAR

STRUCTURE



---