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 1

Very 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 orbitals

covalent 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 p bond

Weaker as compared to s 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 pbonds

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)

BeBe 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)

BB 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"