Covalent bonding Revision Notes for AQA GCSE Chemistry

Covalent bonding

Covalent bonds are a strong type of chemical bond that forms between non-metal atoms. These bonds happen when atoms share electrons with each other to fill their outer shells.

What are covalent bonds?

When non-metal atoms get close together, they can share pairs of electrons. This sharing creates a covalent bond between the atoms. Each shared pair of electrons counts as one covalent bond.

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The key to understanding covalent bonding is that atoms are trying to achieve a stable electronic configuration by sharing electrons rather than transferring them completely (as in ionic bonding).

Covalent bonds can be found in:

Small molecules - like water and methane
Polymers - long chains of repeating units
Giant covalent structures - like diamond and graphite

Elements with small molecules

Some elements exist as small molecules held together by covalent bonds:

Hydrogen ( $H_2$ ) - two hydrogen atoms share one pair of electrons (single bond)
Chlorine ( $Cl_2$ ) - two chlorine atoms share one pair of electrons (single bond)
Oxygen ( $O_2$ ) - two oxygen atoms share two pairs of electrons (double bond)

Each atom in these molecules gets a full outer shell by sharing electrons.

Compounds with small molecules

Non-metal elements can also form compounds by sharing electrons:

Hydrogen chloride ( $HCl$ ) - hydrogen and chlorine share electrons
Water ( $H_2O$ ) - oxygen shares electrons with two hydrogen atoms
Methane ( $CH_4$ ) - carbon shares electrons with four hydrogen atoms

Dot-and-cross diagrams

These diagrams help us see how electrons are shared in covalent bonds. Understanding the symbols used is essential for success:

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How to read dot-and-cross diagrams:

Dots represent electrons from one atom
Crosses represent electrons from another atom
Circles show the outer shell of each atom
Lone pairs are electron pairs not used in bonding

The diagrams only show outer shell electrons. Each atom ends up with a full outer shell after bonding.

Types of covalent bonds

The number of electron pairs shared determines the bond type:

Single bond - one pair of electrons shared (like in $H_2$ )
Double bond - two pairs of electrons shared (like in $O_2$ )
Triple bond - three pairs of electrons shared (like in $N_2$ )

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Worked Example: Nitrogen molecule

Nitrogen atoms are in Group 5, so each has 5 outer electrons. To get a full outer shell of 8 electrons, two nitrogen atoms share 3 pairs of electrons. This creates a triple covalent bond. The formula for nitrogen gas is $N_2$ .

Step-by-step:

Each N atom has 5 outer electrons
Each N atom needs 8 electrons for a full shell
Each N atom needs 3 more electrons
Two N atoms share 6 electrons (3 pairs) between them
Result: $N \equiv N$ (triple bond)

Common exam questions

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Key exam topics you should prepare for:

You might be asked to:

Draw dot-and-cross diagrams for simple molecules
Identify the type of bonding in a molecule
Give the formula of a covalent compound
Explain how atoms achieve full outer shells

Common examples include ammonia ( $NH_3$ ) and tetrachloromethane ( $CCl_4$ ).

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Key Points to Remember:

Covalent bonds form when non-metal atoms share electrons
Each shared pair of electrons makes one covalent bond
Atoms share electrons to get full outer shells
Dot-and-cross diagrams show how electrons are shared
Lone pairs are electrons not used in bonding

Covalent bonding (AQA GCSE Chemistry): Revision Notes