Molecular PolarityThere are two factors that determine if a molecule is polar:

1)Is the bond polar?

If the bond isn’t polar the molecule can’t be polar

Look at the EN difference between the elements

2)Is the overall molecule polar?

Look at the molecular geometry

Example 1: CO2

First look at the bond

C - O

3.5 - 2.5 = 1.0

Bond is polar

Draw dipole

Draw Lewis structure

C OOx x

x x x x

x x

x x

x x

What shape?

Linear

Draw dipoles

Dipoles cancel, thus the molecule is non-polar

Example 2: H2OFirst look at the bond

H - O

3.5 - 2.1 = 1.4

Bond is polar

Draw dipole

Draw Lewis structure

What shape?

Bent

Draw dipoles

The O end is negative and the H end is positive, thus

it is polar

H Oxx H

OH H

δ-

δ+

CF4

C - F

4.0 - 2.5 = 1.5

Cxx

x xxx

x x

Fxx

xxx

x x

F

xx x

xx x

Fx

x xxx

xx F

Tetrahedral

C

F

FFF

Dipoles cancel, thus the molecule is non-polar

NH3

N - H

3.0 - 2.1 = 0.9

Nxx

xH

H

H

N

HHH

δ-

δ+

The N end is negative and the H end is positive, thus

it is polar

What to look for in molecular geometry

Look for symmetry

If the molecule is symmetrical then it will be non-polar

Thus a molecule will be non-polar if the bond is non-polar or if it is symmetrical

To be polar the bond must be polar and the molecule must be asymmetrical

Forces that hold substances together

1. Ionic bond

2. Covalent network

3. Intermolecular forces

A) Dipole-Dipole interaction

B) Hydrogen bond

C) London forces

Ionic Bonds

Held together by ionic bonds - the attraction between oppositely charged ions

Form into a lattice in which each ion is surrounded by several oppositely charged ions

Very strong bonding

Will all be solids at room temperature and will have very high melting and boiling points

Covalent Networks

Example: Diamond

Carbon atoms form a network that is connected by covalent bonds

Very strong bonding

Will be solids at room temperature and will have very high melting and boiling points

Will also be very hard

Other example: SiO2 which makes up Quartz

Covalent substancesConsist of groups of atoms connected by a covalent bond called a molecule

Forces of attraction between molecules are called intermolecular forces

These forces are much weaker than ionic bonds or covalent network forces

The melting and boiling points will therefore be much lower

Many of them will be gases or liquids at room temperature

Dipole-Dipole interactionsEx: HCl

H - Cl

3.0 - 2.1 = 0.9

δ-δ+

H - Clδ-δ+

Line up another molecule

H - Clδ-δ+

Cl - Hδ- δ+

Cl - Hδ- δ+

Covalent bond very strong, takes lots of

energy to break

Dipole-Dipole interaction,

much weaker and takes less

energy to break

Hydrogen Bond

H - O

3.5 - 2.1 =1.4Leaves behind a relatively

unshielded p+

p+bonds to a lone pair on another

molecule

This is called a hydrogen bond

It only forms if the atom that H bonds to has a very high EN

Forms when H is bonded with F, O and N

H2OO

H Hp+ p+

OH H

H Bond

This is why water has very strong cohesion

London forces

What if the substance is non-polar?

What can hold the substance together?

Why would the particles be attracted to each other?

Example: Helium

Boiling point = -269 ᵒC

How can He be a liquid?

He

2 p+

What if the 2 e- are on the same side of

the atom?

Forms a instantaneous dipole

He

2 p+

Induces a dipole of the other He

δ-δ+ δ-δ+

Called an instantaneous induced dipole

Or London force

London forces

The strength increases as the number of e- increases

I2 has strong enough London forces to be a solid at room temperature

What type of attraction

First: look for covalent networks and H bonding

2nd: classify the bond

If it is ionic it is ionic

If it is non-polar covalent then it is held by London forces

3rd: if the bond is polar then look to see if the molecule is polar

Example: What force of attraction is found in CaO? Is it weak or

strong?

Ca

Second: classify the bond

O1.0 3.5

2.5

Ionic

First: look for covalent networks or H bonds

Isn’t C or SiO2 not a network

Doesn’t have H - not H bonding

Strong force

Example: What force of attraction is found in CH3OH? Is it weak or

strong?

Not a network

Has H bonded to O

H bonding Weak force

Example: What force of attraction is found in CH4? Is it weak or

strong?Not a network

Has H - but not bonded to O, N or F

C H2.5 2.1

0.4 Polar bond

Now look at the molecule

Lewis structure

Cxx

x

xH

H

H

H

Tetrahedral

C

HHH

H

It is symmetrical - so nonpolar molecule

London forces

Weak force

Example: What force of attraction is found in PCl3? Is it weak or

strong?Not a network No H

P Cl2.1 3.0

0.9 Polar bond

Lewis structure Triangular pyramidal

Pxx

xCl

Cl

Cl P

ClClCl

δ-

δ+

Asymmetrical - so polar molecule

Dipole-dipole interaction

Weak force

P

P

x

x

x

xx x

Cl

x

ClCl

Cl

Cl

ClC

HHH

P

ClClCl

H