Covalent bonding

What you need to know

• A covalent bond is a strong bond formed when atoms share a pair of electrons.
• Covalently bonded substances can be small molecules, very large molecules such as polymers, or giant covalent structures.
• Dot-and-cross diagrams show shared electrons, while a line can represent a single covalent bond.
• You must be able to read models and diagrams, deduce molecular formulae, represent polymer repeating units and recognise the limitations of different models.

The big picture

Covalent bonds form when atoms share pairs of electrons, and the bonds between the atoms are strong. Covalently bonded substances can exist as small molecules, very large molecules such as polymers, or giant covalent structures such as diamond and silicon dioxide. Bonding can be represented using dot-and-cross diagrams or lines, with one line representing one single covalent bond. Different models show different features, so you also need to understand what each representation leaves out.

From shared electrons to a structure

Follow the same bonding idea through the different ways covalent structures are represented.

1. 1

   Atoms share electrons

   When atoms share a pair of electrons, that shared pair forms a strong covalent bond between the atoms.

2. 2

   Dot and cross shows the pair

   A dot-and-cross diagram uses dots and crosses to distinguish electrons and shows shared pairs between bonded atoms.

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3. 3

   A line can replace a bond

   When the bonding is represented more simply, one line between atoms stands for one single covalent bond.

4. 4

   The structure can grow

   Covalently bonded substances may be small molecules, very large molecules such as polymers, or giant covalent structures.

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5. 5

   The diagram becomes information

   Once the atoms and bonds are shown, count each type of atom to deduce the molecular formula of the molecule.

Tap any cell to reveal it. Tap a column header to read one property down every item.

Representing polymers

Follow the move from one covalent bond to a repeating unit.

1. 1

   Show the bonded atoms

   Lines can represent the single covalent bonds between atoms in a polymer repeating unit.

2. 2

   Mark one repeating unit

   A displayed polymer formula focuses on the part of the very large molecule that repeats.

3. 3

   Attach the n

   The repeating-unit formula uses n to show that the unit occurs a large number of times.

Covalent bonding language

Say the idea before revealing each card.

Memory hook

Share, show, scale: share electrons, show the bonds, then recognise whether the structure is small, polymer-sized or giant.

Check yourself

Why might you choose a dot-and-cross diagram rather than a three-dimensional model of the same molecule?