1

Molecular Geometry and Polarity

Part I: Molecular Geometry – Valence Bond Theory (A)

Dr. Chin Chu

River Dell Regional High School

2

3

Molecular Geometry• Lewis structures tell us how the atoms are

connected to each other, a.k.a. bonding between atoms.

• They do not address how those bonds are formed.

• They also don’t tell us anything about shape.

• The shape of a molecule can greatly affect its properties.

4

Molecular GeometryThree theories:• VSEPR: Valence shell electron pair repulsion

theory– Does not explain in detail how those covalent bonds

are formed between atoms.

• Valence Bond Theory: – Does provide insights into details of covalent bond

formation and associated geometry.

• Molecular Orbital Theory: advanced course

5

Valence Bond Theory

Combines bonding with geometry.Addresses the equality of bonds in a molecule as far as length and energy

are concerned.

6

Valence Bond Theory• Explains the structures of covalently bonded

molecules– ‘how’ bonding occurs

Principles of VB Theory:Bonds form from overlapping atomic orbitals and electron pairs are shared between two atoms

A new set of hybridized orbitals may form out of necessity.

Lone pairs of electrons are localized on one atom

7

http://csi.chemie.tu-darmstadt.de/ak/immel/script/redirect.cgi?filename=http://csi.chemie.tu-darmstadt.de/ak/immel/tutorials/orbitals/vrml.html

• Formation of any chemical bonds is driven by the desire of atoms to achieve valence electron configurations with lower energy (more stable) state, a.k.a. Octet Rule.

• For example: chlorine gas, Cl2

8

How Do Covalent Bonds Form?

Cl Cl

Each chlorine atom has 7 valence e-, one short of

the desirable 8 e-.

Where will the extra e- coming from?Hey, the other chlorine

atom. You got some valence e-, right? How

about give me one?

OK, you don’t have to give me an attitude. How about we share one e- so

each of us can get 8?

Cl ClThat is win-win.

Everyone is happy. Hence the formation of a

single covalent bond between two Cl atoms.

• Wait! According to Schrodinger’s quantum mechanical theory, electrons must be on specific orbitals, and there are maximum two electrons occupying the same orbital spinning in opposite directions (Pauli exclusion principle).

• What actually happened when the Cl-Cl bond is formed?

9

How Do Covalent Bonds Form?

Clatom

ground state valence shell

e- conf.

3s

3p Cl

z

y

xCl

ClCl Cl

10

How Do Covalent Bonds Form?

• In VBT, a bond will be formed if there is overlap of appropriate orbitals on two atoms and these orbitals are populated by a maximum of two electrons.

• The two 3p orbitals occupied by 2 electrons are not going to be involved in bonding due to limit imposed by the Pauli Exclusion Principle.

• But the one 3p orbital occupied by 1 electron is capable of forming a strong bond with the other 3p orbital from the 2nd Cl atom.

11

VBT: Orbital Overlap to Form Sigma Bonds

12

Sigma Bonds

bonds: symmetric about the internuclear axis

s-s overlap

s-p overlap

p-p overlap: Sideways overlap

• Bonding in Oxygen Molecule (O2)

• Bonding in Nitrogen Molecule (N2)

13

Valence Bond Theory: Pi Bond

14

Valence Bond Theory: Pi Bond

bonds: form by vertical overlap. Exist in double and triple bonds

Atomic axis

15

Forms by a vertical overlap of two p orbitals (p-p vertical overlap)Exists only when there are multiple covalent bondsExample H2 -C=C-H2: the double bonds contains one sigma and one pi bond

Formation of Pi Bond

16

Types of Overlaps

• Sigma bond (σ ) bonds-s overlapp-p overlaps-p overlap

• sp overlap• sp2

• sp3

• sp3d• sp3d2

• Pi (π) bond