Covalent bonding is where students most often lose “easy” marks, usually for one of two reasons: miscounting electrons in a dot-and-cross diagram, or explaining a low boiling point by saying that covalent bonds break. This set targets both. It covers counting bonds and lone pairs, multiple bonds, the difference between simple molecular and giant covalent structures, polarity and basic molecular shape.
There are 15 questions worth 44 marks. Diagrams are described in words here, but on paper you should draw them: circles for outer shells, dots for one atom’s electrons, crosses for the other’s, with the shared pairs sitting where the circles overlap. Give each question a proper attempt before you look at the answer key.
Useful information
Outer-shell electrons: H 1, C 4, N 5, O 6, Cl 7. Pauling electronegativities: H 2.20, C 2.55, N 3.04, Cl 3.16, O 3.44.
Questions
1. State what is meant by a covalent bond. (2 marks)
2. State the number of covalent bonds normally formed by an atom of (a) carbon, (b) nitrogen, (c) oxygen, (d) chlorine. (4 marks)
3. Describe the dot-and-cross diagram for a chlorine molecule, Cl₂, showing outer electrons only. (2 marks)
4. Describe the dot-and-cross diagram for water, H₂O. State how many lone pairs the oxygen atom has. (3 marks)
5. In ammonia, NH₃, state the number of bonding pairs and lone pairs around the nitrogen atom. (2 marks)
6. An oxygen molecule, O₂, contains a double bond. Describe its dot-and-cross diagram and state how many electrons are shared. (3 marks)
7. Nitrogen, N₂, contains a triple bond. How many electrons are shared between the two atoms, and how many lone pairs does each nitrogen atom have? (2 marks)
8. Describe the dot-and-cross diagram for carbon dioxide, CO₂. How many electrons are in the outer shell of the carbon atom in the molecule? (3 marks)
9. Which of these compounds are covalent? NaF, CH₄, KBr, SO₂, HCl, MgCl₂. (3 marks)
10. Methane is a gas at room temperature even though the C–H bond is strong. Explain why. (3 marks)
11. Explain why simple molecular substances such as sulfur and iodine do not conduct electricity. (2 marks)
12. Diamond and methane both contain only covalent bonds, yet diamond has a very high melting point. Explain the difference. (4 marks)
13. Graphite is a form of carbon that conducts electricity. Explain why, with reference to its structure. (3 marks)
14. (a) Using the electronegativity values, show which bond is more polar: H–Cl or O–H. (2 marks) (b) State which atom carries the partial negative charge (δ−) in HCl. (1 mark) (c) Explain why the C–H bond is often treated as almost non-polar. (1 mark)
15. Name the shape and give the bond angle of (a) methane, CH₄, and (b) water, H₂O. (4 marks)
Total: 44 marks
Answer key
1. Definition (2 marks)
A covalent bond is a shared pair of electrons between two atoms (1), held in place by the electrostatic attraction between the two nuclei and the shared electrons (1). Common error: saying “atoms share electrons” without “pair” usually drops the first mark.
2. Number of bonds (4 marks, 1 each)
Count how many electrons each atom needs to reach eight in its outer shell. (a) Carbon, 4 outer electrons, needs 4 → 4 bonds (b) Nitrogen, 5, needs 3 → 3 bonds (c) Oxygen, 6, needs 2 → 2 bonds (d) Chlorine, 7, needs 1 → 1 bond
3. Chlorine, Cl₂ (2 marks)
- Two overlapping circles with one shared pair (one dot and one cross) in the overlap (1).
- Each chlorine also has three non-bonding pairs (six electrons of its own) outside the overlap, giving each atom eight outer electrons (1).
4. Water, H₂O (3 marks)
- Oxygen in the centre, overlapping with two hydrogen circles; each overlap contains one dot and one cross (1).
- Oxygen has four further electrons of its own, drawn as two lone pairs (1).
- Two lone pairs on oxygen (1).
Check: oxygen has 6 of its own + 2 from hydrogen = 8. Each hydrogen has 2, which is a full shell for hydrogen. If you want more help with these diagrams, work through dot-and-cross diagrams.
5. Ammonia, NH₃ (2 marks)
Three bonding pairs (1) and one lone pair (1). Working: nitrogen has 5 outer electrons; 3 are used in bonds with hydrogen, and the remaining 2 form one lone pair.
6. Oxygen, O₂ (3 marks)
- Two shared pairs in the overlap, each made of one dot and one cross (1).
- Each oxygen has two lone pairs outside the overlap (1).
- Four electrons are shared (1).
Working: each oxygen has 6 outer electrons and needs 2 more, so each contributes 2 electrons to the bond. 2 + 2 = 4 shared.
7. Nitrogen, N₂ (2 marks)
Six electrons (three shared pairs) (1). One lone pair on each nitrogen (1). Working: nitrogen has 5 outer electrons and uses 3 in the triple bond, leaving 2 = one lone pair. Check: 6 shared + 2 own = 8 for each atom.
8. Carbon dioxide, CO₂ (3 marks)
- Carbon in the centre with two shared pairs (a double bond) to each oxygen (1).
- Each oxygen has two lone pairs (1).
- Carbon has 8 electrons in its outer shell: 4 of its own and 4 from the two oxygens (1).
Common error: drawing single bonds to each oxygen. That leaves carbon with only 6 electrons and each oxygen with only 7.
9. Covalent compounds (3 marks, 1 each)
CH₄, SO₂ and HCl. All three are made only of non-metal atoms. NaF, KBr and MgCl₂ each contain a metal combined with a non-metal and are ionic. Be careful with guessing on a question like this: mark schemes often cancel one correct answer for every wrong one you add.
10. Methane is a gas (3 marks)
- Methane is a simple molecular substance (1).
- The forces between molecules (intermolecular forces) are weak (1).
- Only a small amount of energy is needed to overcome them, so methane boils at a very low temperature; the covalent bonds inside the molecules do not break when it boils (1).
This is the single most common mistake in the topic. Boiling methane gives methane gas, not carbon and hydrogen.
11. Non-conduction (2 marks)
- Simple molecules have no overall charge (1).
- There are no free electrons or ions to carry a current (1).
12. Diamond versus methane (4 marks)
- Diamond has a giant covalent structure (1).
- Each carbon atom is covalently bonded to four others in a continuous network (1).
- To melt diamond, many strong covalent bonds must be broken, which needs a very large amount of energy (1).
- Methane is simple molecular; melting only overcomes weak intermolecular forces, not covalent bonds (1).
For a full comparison of the two structure types, see giant covalent structures and the element page for carbon.
13. Graphite (3 marks)
- Each carbon atom is covalently bonded to three other carbon atoms, forming layers of hexagons (1).
- This leaves one outer electron per carbon atom that is not used in a bond (1).
- These electrons are delocalised and can move along the layers, carrying a current (1).
14. Polarity (4 marks)
(a) H–Cl: 3.16 − 2.20 = 0.96. O–H: 3.44 − 2.20 = 1.24 (1). The larger difference means O–H is more polar (1).
(b) Chlorine (δ−), because it has the higher electronegativity and pulls the shared pair towards itself; hydrogen is δ+ (1).
(c) C–H: 2.55 − 2.20 = 0.35. The difference is very small, so the electrons are shared almost equally (1).
The polar covalent bonds post shows how bond polarity leads to polar and non-polar molecules.
15. Shapes (4 marks)
(a) Methane: tetrahedral (1), 109.5° (1). Four bonding pairs repel each other equally and get as far apart as possible in three dimensions.
(b) Water: bent (also called V-shaped or non-linear) (1), about 104.5° (1). There are still four pairs around oxygen, but two are lone pairs, which repel more strongly than bonding pairs and push the two O–H bonds closer together.
Common error: drawing water as a straight line (180°). The two lone pairs are the reason it is bent.
Score guide
- 37–44: well prepared. Stretch yourself with the Lewis structures practice set.
- 26–36: solid, but look again at any diagram questions you missed; recount electrons around every atom.
- Below 26: rework questions 2–8 slowly. Most errors come from counting, not from understanding.
If you have not tried it yet, the matching ionic bonding practice set covers the other half of this topic.
Summary
Every covalent diagram rests on one check: count the outer electrons around each atom, including shared pairs, and make sure it reaches eight (or two for hydrogen). The number of bonds an atom forms equals the number of electrons it needs, and any outer electrons left over appear as lone pairs. Multiple bonds share two or three pairs. When it comes to properties, always separate the two kinds of force: strong covalent bonds within molecules, weak intermolecular forces between them. Simple molecular substances melt and boil easily because only the weak forces are overcome; giant covalent structures such as diamond melt at very high temperatures because covalent bonds must break. Polarity comes from electronegativity difference, and shape comes from electron-pair repulsion, with lone pairs pushing harder than bonding pairs.
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