AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Bonding, structure, and the properties of matter › Chemical bonds, ionic, covalent and metallic
Practise Covalent bonding. 10 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How non-metal atoms share pairs of electrons to form covalent bonds, the three kinds of covalent substance (small molecules, very large molecules and giant structures), and the different ways of drawing them. You must be able to draw dot and cross diagrams for eight small molecules and work out formulae from diagrams.
Key facts
Covalent bond = a shared pair of electrons between two non-metal atoms (strong)
Bonds usually formed: H 1, Cl 1, O 2, N 3, C 4
O2: double bond, O=O. N2: triple bond, N≡N
Non-bonding pairs: each Cl in Cl2 3; O in H2O 2; N in NH3 1; C in CH4 0
Covalent substances: small molecules, very large molecules (polymers), giant covalent structures
Displayed formula: one line = one covalent bond
Molecular formula = the actual numbers of atoms in one molecule (do not simplify)
Notes
Sharing pairs of electrons
When non-metal atoms bond, they share pairs of electrons. Each shared pair is one covalent bond. Covalent bonds are strong.
Atoms share electrons until each has a full outer shell. The number of bonds an atom usually forms is the number of electrons it needs: H and Cl form 1, O forms 2, N forms 3, C forms 4.
Two shared pairs make a double bond (O=O); three shared pairs make a triple bond (N≡N).
The eight dot and cross diagrams to learn
diagram
Hydrogen, H2: one shared pair. Chlorine, Cl2: one shared pair, plus three non-bonding pairs (lone pairs) on each Cl.
Hydrogen chloride, HCl: one shared pair, plus three non-bonding pairs on the Cl.
Water, H2O: two shared pairs (one to each H), plus two non-bonding pairs on the O.
Ammonia, NH3: three shared pairs, plus one non-bonding pair on the N. Methane, CH4: four shared pairs and no non-bonding pairs.
Outer shells only. Each shared pair (one dot, one cross) is one covalent bond.
Oxygen, O2: two shared pairs, plus two non-bonding pairs on each O. Nitrogen, N2: three shared pairs, plus one non-bonding pair on each N.
Two shared pairs make a double bond; three make a triple bond.
Check: every atom except hydrogen ends with 8 outer electrons; each hydrogen has 2.
Three kinds of covalent substance
Small molecules: a few atoms joined by covalent bonds, e.g. the eight above and CO2.
Very large molecules, such as polymers: long chains of atoms.
Giant covalent structures: huge networks in which every atom is covalently bonded, e.g. diamond and silicon dioxide.
Ways of drawing covalent substances
Dot and cross: shows the shared pairs and which atom each electron came from, but not the 3D shape.
Displayed formula: each covalent bond is a line, e.g. H–O–H. It shows which atoms are bonded, but not the non-bonding electrons or the 3D shape. Lines are also used for polymer repeating units and giant structures.
Ball and stick (3D): shows the shape, but bonds are not really sticks, and the atoms are really much closer together and not to scale.
A molecular formula gives the actual number of each type of atom in one molecule. Count them from the diagram and do not cancel down: ethane is C2H6, not CH3.
How to answer each type of question
Draw a dot and cross diagram for a molecule
2 marksGrade 5
Work out how many bonds each atom needs to fill its outer shell.
For each bond, put one shared pair (a dot and a cross) where the atoms overlap.
Add non-bonding pairs so that every atom except hydrogen has 8 outer electrons.
Check that each atom still has its own original number of outer electrons.
Example. Draw a dot and cross diagram for a molecule of ammonia, NH3. Show the outer shell electrons only.
Show the model answerHide the model answer
The diagram should show: three shared pairs, each one dot and one cross, one between the N and each H (1) one non-bonding pair on the nitrogen, so the nitrogen has 8 outer electrons in total (1)
Don’t lose marks
Drawing a shared pair with only one electron, or with both electrons from the same atom. Each covalent bond here has one electron from each atom.
Forgetting the non-bonding pairs on O, N and Cl.
Drawing a single bond in O2 or N2.
Simplifying a molecular formula, e.g. writing CH3 for ethane.
Saying covalent bonds are weak. The bonds are strong; the forces between molecules are weak.
More tips
Memory tricks
HONC 1234: hydrogen forms 1 bond, oxygen 2, nitrogen 3, carbon 4. Chlorine, like hydrogen, forms 1.
Bonds needed = 8 − number of outer electrons (for atoms in Groups 4 to 7); hydrogen needs just 1 more to reach 2.
Quick check on any dot and cross diagram: count the electrons around each atom. It should be 8, or 2 for hydrogen.
Molecular formula: count, don't cancel.
Exam technique
Draw overlapping circles for bonded atoms and put the shared pairs in the overlap. Examiners check the number of electrons in each region.
The question usually says 'outer shell only'. Don't waste time drawing inner shells, but if you do, make sure they are correct.
For a limitation, name the specific thing the diagram fails to show (3D shape, relative sizes, where the electrons came from), not just 'it's not realistic'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Describe a covalent bond as a shared pairA covalent bond is a pair of electrons shared between two atoms. Covalent bonds are strong.
Grade 4
Recognise small molecules from their formulaeFormulae of a few non-metal atoms, e.g. H2, Cl2, HCl, H2O, NH3, CH4.
Grade 5
Draw dot and cross diagrams for simple moleculesHydrogen, chlorine, hydrogen chloride, water, ammonia and methane, which have single bonds only.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
How many covalent bonds are there in one molecule of methane, CH4?
4
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Covalent bonds form between non-metal atoms.
(a) What is a covalent bond? Tick (✓) one box.[1]
A shared pair of electrons between two atoms
The attraction between oppositely charged ions
The attraction between positive ions and delocalised electrons
A weak force between molecules
(b) Which substance consists of small molecules? Tick (✓) one box.[1]
Ammonia
Diamond
Poly(ethene)
Sodium chloride
(c) How many covalent bonds are there in one molecule of methane, CH4?[1]
(d) Silicon dioxide contains covalent bonds. What type of structure does silicon dioxide have? Tick (✓) one box.[1]
Giant covalent
Giant ionic
Giant metallic
Small molecules
Show the answer and mark scheme
(a)Answer: A shared pair of electrons between two atoms
(b)Answer: Ammonia
(c)Answer: 4
4
(d)Answer: Giant covalent
Question 2Medium5 marks
Dot and cross diagrams show how the outer electrons are arranged in molecules.
(a) How many pairs of electrons are shared in a molecule of ammonia, NH3?[1]
(b) How many non-bonding pairs of electrons are there on the oxygen atom in a molecule of water, H2O?[1]
(c) Explain why a hydrogen atom forms only one covalent bond.[1]
(d) Describe the dot and cross diagram for a molecule of hydrogen chloride, HCl.[2]
Show the answer and mark scheme
(a)Answer: 3
3
(b)Answer: 2
2
(c)Answer: It has only one electron; sharing one pair gives it a full first shell (2 electrons).
hydrogen has one electron / its (first) shell is full with 2 electrons after sharing one pair
(d)Answer: One pair of electrons is shared between H and Cl (one electron from each); the chlorine atom also has three non-bonding pairs.
one pair of electrons shared between H and Cl (one electron from each atom)
the chlorine atom has three non-bonding pairs / 8 electrons in its outer shell