Covalent Bonding

Covalent Bonding

20 April 2023

By the end of this lesson, you should be able to….

• Describe and define a covalent bond

• Describe single and multiple covalent bonding

• Use dot and cross diagrams to represent covalent bonding

Covalent Bonds

Only occurs between non-metals

Formed by a shared pair of electrons

Why is there an attraction?

A covalent bond is directional, it only acts on the atoms involved, unlike an ionic bond

Hydrogen and Hydrogen

HH2

H

Click for another exampleClick for animation

Nitrogen and Hydrogen (Ammonia)

Click for animationClick for another example

H

H

HN

NH3

Hydrogen and Oxygen

Click for animation

H2O

H H

O

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TaskThis is the dot-and-cross diagram for a water molecule. Draw dot-and-cross diagrams for the following:

1.F2

2.H2

3.HCl

4.CH4

5.SCl2

Lone pairsWhen an electron pair is not used for bonding, it is known as a lone pair.

A water molecule has 2 lone pairs.

An ammonia molecule has 1 lone pair.

Lone pairs can affect the chemistry of molecules in quite significant ways.

Multiple covalent bonds

Some atoms can share more than one pair of electrons to form a multiple bond.

O O

Carbon Dioxide

O OC

TaskDraw dot and cross diagrams for the

following:

a) C2H4

b) HCΞN

c) H2C=O

Naming covalent compounds

Two nonmetals– Name each element – End the last element in -ide– Add prefixes to show more

than 1 atomPrefixes

mon 1 penta 5di 2 hexa 6tri 3tetra 4

Naming Covalent CompoundsFill in the blanks to complete the following

names of covalent compounds.

CO carbon ______oxide

CO2 carbon _______________

PCl3 phosphorus _______chloride

CCl4 carbon ________chloride

Answers

CO carbon monoxide

CO2 carbon dioxide

PCl3 phosphorus

trichloride

CCl4 carbon

tetrachloride

Dative Bonding

A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.

Lone pair

Dative BondingYou can represent a dative covalent bond with an arrow, AB

The direction of the arrow, shows the direction that the electron pair has been donated.

Dative BondingWhen an acid is added to water, oxonium ions can form (H3O+)

Dative Bonding

Draw dot-cross diagrams for:

a)PCl4+

b)H3O+

c)H2F+

Complications……(as usual)When covalent bonds form, unpaired

electrons pair up to obey the Octet Rule, however this isn’t always possible because:

• there may not be enough available electrons to reach an Octet

• there may be more than four electrons that pair up during bonding – this is called ‘Expansion of the Octet’

Not enough electrons….Eg: Be and B both form compounds with covalent

bonds (despite being metals) – they do not have enough unpaired electrons to reach an octet……

Expansion of the octetIn groups 5-7 something unusual happens.Moving down the groups means that more of

their outer-shell electrons are able to take part in bonding – this can break the Octet Rule

e.g. phosphorous can form two chlorides, PCl3 and PCl5 it just depends on how much chlorine is available.

Task

Draw dot-cross diagrams for the following compounds:

1. BF3

2. PF5

3. SO2

4. SO3

Extension:5. BF3NH3 (This compound is formed when BF3

and NH3 react together)

Shapes of MoleculesNow we know how covalently bonded compounds

form we can look at their shape. On paper they look flat, but many are 3D.

How to predict the shape of a molecule:1. Draw the dot and cross diagram2. Count the number of electrons surrounding the

central atom3. Divide this by 2 to find out the number of

electron pairs.These will arrange themselves so that they are as

far apart as possible – REMEMBER: They are negative and so repel

Use the molymods to make the following 3D structures

CH4

PCl3

NH3

CO2

SF6

NO2+

Describing shapes of molecules

Bond length is the distance between the nuclei of two bonded atoms.

Bond angle is the angle between two covalent bonds.

bond angle

bond length

Counting electrons enables the basic shape of the molecule and its approximate bond angles to be predicted.

The shape of a molecule can be described in terms of its bond lengths and bond angles.

How to draw 3D molecules

For the following covalent compounds do the following:

1. Draw the dot-cross diagram

2. Predict and draw the shape of the molecule

3. Predict the bond angle

CH4

BeCl2

BF3

NH4+

SF6

SiF4

Bonding and lone pairsA pair of electrons in a covalent bond are called a bonding pair. Pairs of electrons that are not involved in bonding are called lone pairs.

Electron pairs are clouds of negative charge, so there is mutual repulsion between them, forcing them as far apart as possible.

This means the number of electron pairs around the central atom(s) determines the basic shape of the molecule.

lonepair

bondingpair

Effect of lone pairs on shapeThe number of lone pairs in a molecule is calculated by subtracting the number of bonding pairs from the total number of electron pairs in the outer principal energy level.

The shape of a molecule with lone pairs is based on the basic shape for the total number of outer electron pairs, but with a lone pair replacing one of the bonds.

tetrahedral pyramidal V-shaped

replacing one bonding pair

with a lone pair

replacing another bonding pair with

a lone pair

Effect of lone pairs on bond angles

QUESTIONSQUESTIONS

For each of the following ions/molecules, state the number of bond pairs, state the number of lone pairs, state the bond angle(s), state, or draw, the shape

SiCl4

PCl6-

H2S

SiCl62-

PCl4+

XeF4

Nitrogen and Boron can form the chlorides NCl3 and BCl3.

a) Draw dot cross diagrams to show the bonding in both NCl3 and BCl3.

b) Draw the shapes of both of these molecules. Show the approximate values of the bond angles and the name of the shape.

c) Explain why the shape of both of these molecules is different.

Effect of lone pairs on bond angles

Strength of metallic bonding: ion chargeThe strength of metallic bonding depends on two factors:

1. the charge on the metal ions

1. The charge on the metal ionsThe greater the charge on the metal ions, the greater the attraction between the ions and the delocalized electrons, and the stronger the metallic bonds. A higher melting point is evidence of stronger bonds in the substance.

2. the size of the metal ions.

Na Mg Al

1+ 2+ 3+

371 923 933

Element

Charge on ion

Melting point (K)

Strength of metallic bonding: ion size

Element

Ionic radius

(nm)Melting point (K)

Li Na K Rb Cs

0.076 0.102 0.138 0.152 0.167

454 371 337 312 302

2. The size of the metal ionsThe smaller the metal ion, the closer the positive nucleus is to the delocalized electrons. This means there is a greater attraction between the two, which creates a stronger metallic bond.

Types of bonding

substances:

sodium chloride

aluminium

water

iodine

diamond carbon

graphite carbon

hydrogen chloride

Discuss the bonding, structure and properties of the following substances. Points to consider and an example follow.

NaCl Al H2O I2 C (diamond) C(graphite) HCl

•Which types of chemical bonding are present within the substance in the solid, liquid and gaseous states.

•The name given to the type of substance.

•The nature of the bonding present.

•The arrangement of particles in space within the solid, liquid and gaseous states.

•How strongly the particles are held together.

•What happens to the particles and bonds present as the substance is gradually warmed from below its melting point to above its boiling point.

Points to consider:

Aluminium oxide

Which types of chemical bonding are present within the substance in the solid, liquid and gaseous states.

Ionic bonds are present in the solid and liquid states

No bonds are present in the gaseous state

The name given to the type of substance.

Aluminium oxide is an ionic compound

Aluminium oxide

The nature of the bonding present.

Ionic bonds are the electrostatic attraction between oppositely charged ions

The arrangement of particles in space within the solid, liquid and gaseous states.

In the solid the ions are fixed in place within a crystal lattice

In the liquid the ions can move through the substance but are still held close together

In the gas the ions are completely separate from each other and move completely freely

Aluminium oxide

How strongly the particles are held together.

Ionic bonds are strong attractions

What happens to the particles and bonds present as the substance is gradually warmed from below its melting point to above its boiling point.

In the solid the ions vibrate about fixed points

At the melting point some attractions between ions break

The melting point is relatively high because ionic bonds are strong

Aluminium oxideWhat happens to the particles and bonds present as the substance is gradually warmed from below its melting point to above its boiling point.

In the liquid the bonds between ions are constantly breaking and reforming so that they can move through the substance

The boiling point is relatively high because ionic bonds are strong

In all three states the ions move faster when heated

At the boiling point all ionic attractions break

In the gas the ions move completely freely of each other