Chem Chapter 2

Key knowledge

• The use of Lewis (electron dot) structures, structural formulas and molecular formulas

to model the following molecules: hydrogen, oxygen, chlorine, nitrogen, hydrogen chloride,

carbon dioxide, water, ammonia, methane, ethane and ethene

• Shapes of molecules (linear, bent, pyramidal, and tetrahedral, excluding bond angles)

as determined by the repulsion of electron pairs according to valence shell electron pair repulsion

(VSEPR) theory

• Polar and non-polar character with reference to the shape of the molecule

• The relative strengths of intramolecular bonding (covalent bonding) and intermolecular

forces (dispersion forces, dipole-dipole attraction and hydrogen bonding)

• Physical properties of molecular substances (including melting points and boiling points

and non-conduction of electricity) with reference to their structure and bonding

• The structure and bonding of diamond and graphite that explain their properties

(including heat conductivity and electrical conductivity and hardness) and their suitability

for diverse applications

Chapter 2A – Covalent bonding

What do you

notice about

the properties

of metals,

ionic

compounds

and

non-metals?

Intramolecular bonds hold

covalent molecules

Formed between 2 non-metals

Electrostatic attraction

Electrons are shared

Covalent bond

1. Lewis structure (electon dot diagrams)

2. Structural formula

3. Molecular formula

How to present covalent bonded molecules

Try to present phosphorus fluoride:

Turn to pg 49 of

your textbook for

more examples!

Shapes of molecules

Valence shell electron pair repulsion

theory (VSEPR) is used to predict shapes

of molecules based on the electron pairs

in the valence shell.

Bond pairs:

electrons involved in covalent bond

Lone pairs:

electrons not involved in covalent bond

Lone Pairs determining the final shape?

In the VSEPR theory these lone pairs of electrons

behave in the same way as the single covalent

bonds and repel.

BUT Lone pairs are ignored when determining

the final shape of a molecule

Shapes of Molecules

Electron Pair Repulsion

For methane, with four

single covalent bonds this

repulsion results in a

tetrahedral shape.

Shapes of Molecules

Lone Pairs of Electron

In ammonia there are;

three single covalent bonds

one lone pair

The four electron pairs repel forming a

tetrahedral arrangement.

However, lone pair of electrons are ignored

when determining the FINAL shape.

The three single covalent bonds are

described as having a pyramidal

arrangement

Don’t ignore

me but you

have to!!

Shapes of Molecules

Lone Pairs of Electron

In water molecules there are;

Two lone pairs of electrons

Two single covalent bonds

The four electron pairs repel forming

a tetrahedral arrangement

Lone pair of electrons are ignored.

The two single covalent bonds are

described as having a v-shaped or

bent arrangement

That’s why I

am Mickey

Mouse

Kirby!

Shapes of Molecules

Lone Pairs of Electron

In Hydrogen fluoride molecules there is;

Three lone pairs

One single covalent bond

The four electron pairs repel forming a

tetrahedral arrangement

Lone pair of electrons are ignored.

The single covalent bond is described as

having a linear arrangement.

The more

lone pairs,

the more

…………… ?

Try it yourself:

Predicting the shape of molecules

Predict the shape of a molecule of phosphine (PH3).

Steps

1.

Draw the electron dot diagram.

2.

Count the number of bonds and loan pairs on the central atom

3.

How will the groups of electrons arrange themselves?

4.

Decide the shape, consider the arrangement of only the atoms

Try it yourself:

Predicting the shape of molecules

Predict the shape of a molecule of hydrogen sulfide (H2S).

Steps

1.

Draw the electron dot diagram.

2.

Count the number of bonds and loan pairs on the central atom

3.

How will the groups of electrons arrange themselves?

4.

Decide the shape, consider the arrangement of only the atoms

Double bonds, Triple bonds and VESPR theory

VSEPR theory treats double and triple bonds the same

way that it treats single bonds.

If a central atom has two single bonds and one double

The bonds arrange themselves to repel from each other to get

maximum separation.

Some examples:

Trigonal Planar

Linear

Linear

How does the shape tell the polarity of a molecule?

Non-polar molecules :

Shape is symmetrical

Polar molecules:

Shape is asymmetrical

🡪 A permanent dipole is formed

Try it yourself:

The strength of the forces between the molecules is known as

intermolecular forces.

Intermolecular Forces: Strength

Are 100 times weaker than ionic, metallic and covalent bonds.

When a covalent molecular substance is converted from a solid to a

liquid or a liquid to a gas intermolecular forces are broken.

Chapter 2B –

Intramolecular bonding and Intermolecular forces

Types of Intermolecular Forces

There are many factors that determine the strength

and type of intermolecular forces

Size

Shape

Polarity

There are three main types;

1. Dipole-dipole forces

2. Dispersion forces (a type of dipole-dipole force)

3. Hydrogen bonding

Dipole-dipole forces

Only occur in polar molecules.

The attraction between the positive and negative ends of

the polar molecules

Relatively weak due to the partial charges.

The more polar a molecule is the stronger the

dipole-dipole forces are:

Strength is directly related to the melting and boiling points

Stronger the forces higher the melting and boiling points

Temporary dipole-dipole force

(Dispersion forces)

For both non-polar and polar molecules

2. Permanent dipole-dipole force

(Dipole-dipole forces)

- For polar molecules only

Hydrogen bonding

Special form of dipole-dipole forces.

Only occurs between molecules in which hydrogen is covalently bonded

to an nitrogen, oxygen or fluorine atom. (N,O,F Gang!!)

A significant partial charge is created when H is bonded to one of

these atoms due to:

High electronegativities

Small atomic radii

Hydrogen bonding in water

Water has unique properties.

For most substances the solid form is

denser than the liquid.

However ice floats due to being less

dense than water.

This property can be explained by H

bonding.

One H2O molecule can form H bonds

with 4 neighbouring water molecules.

Molecules

The bonds that can be found in liquid water are;

Intramolecular bonds: bonds between Hydrogen and Oxygen atoms

within water molecules.

Intermolecular bonds: the bonds between water molecules.

Intramolecular bonds are strong compared to the intermolecular

bonds.

Intermolecular bonds are broken when the substance is boiled or melted.

Recap:

Ice verse liquid water

When ice forms the H

bonding arranges the H2O

molecules into a regular

crystal lattice.

H2O molecules in ice are

held further apart than

they are in liquid H2O.

Strength of dispersion forces

The strength of dispersion forces increases as the size of the

molecule increases.

Why might that be?

Larger molecules have a larger number of electrons

Making it easier to produce temporary dipoles

Stronger dispersion forces 🡪 higher melting and boiling points

Strength of dispersion forces

The shape of a molecule also

influences the strength of the

dispersion force.

Butane has a longer chain and is

less compact than

methylpropane.

Being less compact allows butane

to have more contact area to

interact with neighbouring

molecules.

Try it yourself:

Compare these three polar molecules.

Which has the strongest dipole-dipole forces?

Which has the highest boiling point?

Explain how this can occur

How do intermolecular forces affect physical properties?

Molecules in the solid state have low

kinetic energy 🡪 cannot overcome

intermolecular forces that hold them

together.

When there is enough energy to

overcome these forces 🡪 substance

transitions into a liquid, which has

weaker intermolecular forces.

With enough kinetic energy 🡪

molecules can break all

intermolecular forcesto exist as a

gas.

Carbon is a versatile element.

Carbon is tetravalent. (Can form 4

covalent bonds with other elements)

Carbon can exist in pure forms;

diamond and graphite. They exist as

giant atomic lattices, called

macromolecules.

Chapter 2C – Macromolecules

Allotropes

Elements that have their atoms arranged differently are called

allotropes.

They have different physical forms.

Atom bonds differ allowing for different properties

Carbon has many allotropes:

Diamond

Graphite

Amorphous carbon

Graphene

Fullerene (Buckyball)

Structure of diamond

Repeating units of carbon

organised into a 3D

tetrahedral lattice structure

Each carbon atom is bonded

to 4 other carbon atoms

(single covalent bonds)

Can’t conduct electricity

Covalent network lattice

Structure of graphite

Layers of hexagonal 2D carbon

lattices, covalently bonded

Each carbon atom is bonded to 3

other carbon atoms, one unpaired

electron (delocalised) 🡪 Can

conduct electricity

Held by weak dispersion forces

Covalent layer lattice

Properties and application of diamond

Properties and application of graphite

Please do

Chapter 2A, 2B, 2C Questions

Chapter 2 Review Questions (pg 73)