1.7 Covalent Bonding

REVISION NOTES

IGCSE Edexcel Chemistry

1.7.1 Know that a covalent bond is formed between atoms by the sharing of a pair of electrons

RECALL

Nonmetals atoms tend to gain electrons to have full electron shells
When a few nonmetal atoms come together, they tend to share electrons

COVALENT COMPOUNDS

Nonmetal atoms share pairs of electrons
Two or more atoms covalently bonded together to form molecules
There are weak intermolecular forces between individual molecules

For example, in water molecules:

Two hydrogen atoms and one oxygen atoms covalently bonded to form a water molecule
There are weak intermolecular forces between the water molecules

In a molecule:

Electrons shared are called bonding electrons
Electrons on the outer shell that are not shared are called non-bonding electrons
Electrons are all fixed (no free electrons), therefore simple covalent molecules
do not conduct electricity

1.7.2 Understand covalent bonds in terms of electrostatic attractions

COVALENT BONDING

Electrostatic attraction between the positive nuclei of the atoms and the shared pairs of negative electrons

1.7.3 Understand how to use dot-and-cross diagrams to represent covalent bonds in:

Diatomic molecules, including hydrogen, oxygen, nitrogen, halogens and hydrogen halides
Inorganic molecules including water, ammonia and carbon dioxide
Organic molecules containing up to two carbon atoms, including methane, ethane, ethene and those containing halogen atoms.

1.7.4 Explain why substances with a simple molecular structures are gases or liquids, or solids with low melting and boiling points
the term intermolecular forces of attraction can be used to represent all forces between molecules

INTERMOLECULAR FORCES

Covalent bonds are formed in between atoms in a molecule
Intermolecular forces are formed between molecules

STATE OF MATTER

The state of matter of substances depend on how strongly the particles are bonded together
In simple molecular structures, the molecules are bonded together by weak intermolecular forces
Therefore, it is easier to break the intermolecular forces between molecules (not covalent) in boiling or melting
Thus, the boiling and melting point of simple molecules are lower

1.7.5 Explain why the melting and boiling points of substances with simple molecular structures increase, in general, with increasing relative molecular mass

INTERMOLECULAR FORCES INCREASE WITH THE SIZE OF THE MOLECULES

Molecules with greater relative molecular masses are larger
There are more electrons in the molecule to be polarised
Therefore, there are stronger intermolecular forces between the molecules
Larger molecules have higher melting and boiling points

1.7.6 Explain why substances with giant covalent structures are solids with high melting and boiling points

GIANT COVALENT STRUCTURES

They have a huge number of non-metal atoms bonded to other non-metal atoms via strong covalent bonds
These structures can also be called giant lattices
They have a fixed ratio of atoms in the overall structure
Three common macromolecules:
- Diamond
- Graphite
- C₆₀ fullerene

1.7.7 Explain how the structures of diamond, graphite and C₆₀fullerene influence their physical properties, including electrical conductivity and hardness

DIAMOND

Diamond, graphite and C₆₀ fullerene are allotropes of carbon
- Allotropes: each of two or more different physical forms in which an element can exist
In diamond, each carbon atom bonds with four other carbons, forming a tetrahedron
All covalent bonds are strong intramolecular forces
There is no intermolecular force

Properties of Diamond

USES OF DIAMOND

Diamond is used in jewellery and for coating blades in cutting tools
The cutting edges of discs used to cut bricks and concrete are tipped with diamonds
Heavy-duty drill bits and tooling equipment are also diamond tipped

GRAPHITE

Each carbon atom in graphite is bonded to three others
Form layers of hexagons, leaving one free electron per carbon atom
- The free electrons move in between the layers
There are strong intramolecular bonds between carbon atoms
There are also weak intermolecular forces between the layers

Properties of Graphite

USES OF GRAPHITE

It is used in pencils
It can be used as an industrial lubricant, in engines and in locks
It is also used to make inert electrodes for electrolysis, which is particularly important in the extraction of metals such as aluminium

C₆₀ Fullerene

Fullerenes are a group of carbon allotropes in which carbon atoms form hollow tubes or spheres
The first fullerene to be discovered was buckminsterfullerene which is also known as a “buckyball”
In this fullerene, 60 carbon atoms are joined together forming 20 hexagons and 12 pentagons which produce a hollow sphere that is the exact shape of a soccer ball

Uses of C₆₀ fullerene

Fullerenes can be used to trap other molecules by forming around the target molecule and capturing it, making them useful for targeted drug delivery systems
They also have a huge surface area and are useful for trapping catalyst molecules onto their surfaces making them easily accessible to reactants so catalysis can take place
Some fullerenes are excellent lubricants and are starting to be used in many industrial processes