Post on 22-Feb-2016
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CHEMICAL BONDING
1. Introduction
2. Octet rule
3. Different types of bonding
4. Valency Bond Theory
Topics Covered
Force of attraction holdinggroup(s) of atoms
What is a Chemical Bond?Chemical bonds
Na+ Cl-
Better stability against
chemical reagents
But why bonds are formed ??
Atoms
two electrons in the valenceshell (1s2)..He
Octet rule noble gas
configuration to attain betterstability.
Na
2 8 1Very reactive
Na
2 8
+
Ne
Cl
2 8 7
Very reactive
Cl
2 8 8
Ar
-
SFF
F
F
FF*
***
*
* .....
.
In SF6, ‘S’ has twelve electron in itsvalence shell, leads to minimisation of energy.
Other examples are: PCl5, BF3
Limitation of octet rule
Bonding
Ionic
Covalent
Co-ordinate or dative
Metallic
Pi bond Sigma bond
Formation of ionic bond
Covalent bondFormed by
mutual sharing of electrons
Covalent bonds
1 1
H2C CH2
Double bond HC CHTriple bond
1 2
non-polar covalent bond between two carbon atomspolar covalent bond between carbon and hydrogen atoms.
Formation of covalent bond
Covalent bonds are called directional while ionic bonds are called non-directional -explain
Solution:
Illustrative Problem
p and d-orbitals generate directional covalent bond.
electrostatic force of attraction.
Ionic bond
overlap of atomic orbitalscovalent bond
Strength of these sigma bonds is in the order:
sigma bond forms due to end-to-end or head-on overlap
p-p+
s-s
+
+
Orbital Overlap Concept
s-p
p-p > s-p >s-s
This is formed by lateral or sideways overlap which is possible for p or d-orbitals.
Sigma bond is stronger than pi bond due to greater extent of overlap.
+ or
Orbital Overlap Concept
Difference between sigma and pi bonds
Stronger as compared to bond
Weaker as compared to bond
H C C H
Formed by head-on overlapping of s-s or s-p or p-p or any hybrid orbital
Formed by side ways overlapping of unhybridised p-orbital
First bond between any two atoms is always sigma
Rest are bonds
In plane of molecule
Perpendicular to plane of molecule
Valence Bond (VB) Theory, the theory we will explore,describes the placement of electrons into bonding orbitals located around the individual atoms from which they originated.
COVALENT BOND FORMATION (VB THEORY)
In order for a covalent bond to form between two atoms, overlap must occur between the orbitalscontaining the valence electrons.
The best overlap occurs when two orbitals are allowedto meet “head on” in a straight line. When this occurs,the atomic orbitals merge to form a single bonding orbital and a “single bond” is formed, called asigma () bond.
Dotted areas: representation of "electron cloud" for one electron
"Head-on Overlap"
Sigma Bond: merged orbital, 2 e's
MAXIMIZING BOND FORMATION
In order for “best overlap” to occur, valence electronsneed to be re-oriented and electron clouds reshapedto allow optimum contact.
To form as many bonds as possible from the available valence electrons, sometimes separation of electron pairs must also occur.
We describe the transformation process as “orbitalhybridization” and we focus on the central atom in the species...
Hybridization of Be in BeCl2
Atomic Be: 1s2
2s2
2p
2s
Energy
separate 2p
2s
"hybridize"
"sp" "sp"
Hybrid sp orbitals:1 part s, 1 part p
"arrange"
(VSEPR)
Be Be is said to be "sp hybridized"
FORMATION OF BeCl2:
Each Chlorine atom, 1s22s22p63s23p5 , has one unshared electron in a p orbital. The half filled p orbital overlaps head-on with a half full hybrid sp orbital of the beryllium to form a sigma bond.
BeCl Cl
BeCl Cl
BeCl Cl
sp hybridized, linear, 180o bond angles
“sp2” Hybridization: All 3 Region Species
BF3 BF FB 33F 21
24 e's/2= 12 prs
BF F (octet violator)
Number of regions around CENTRAL ATOM: 3
shape : TRIGONAL PLANARbond angles: 120o
F
F
BF F
F
Hybridization of B in BF3
Atomic B : 1s2 2s2 2p1
Valence e’s
2p
2s
Energy
separate 2p
2s
"hybridize"
"sp2" "sp2" "sp2"
Hybrid sp2 orbitals:1 part s, 2 parts p
"arrange"
(VSEPR)
B B is said to be "sp2 hybridized"
FORMATION OF BF3:
Each fluorine atom, 1s22s22p5, has one unshared electron in a p orbital. The half filled p orbitaloverlaps head-on with a half full hybrid sp2 orbitalof the boron to form a sigma bond.
F F
sp2 hybridized, TRIGONAL PLANAR, 120o bond angles
B
F
F F
B
F
Hybridization of C in CH4
Atomic C : 1s2 2s2 2p2
Valence e’s
2p
2s
Energy
separate 2p
2s
"hybridize"
"sp3" "sp3" "sp3" "sp3"
Hybrid sp3
orbitals:1 part s, 3 parts p
FORMATION OF CH4:
Each hydrogen atom, 1s1, has one unshared electron in an s orbital. The half filled s orbitaloverlaps head-on with a half full hybrid sp3 orbitalof the carbon to form a sigma bond.
"arrange"
(VSEPR)C
C is said to be "sp3 hybridized"