Welcome to Module 4 — the final chemistry module! Inorganic chemistry covers bonding (how atoms stick together), states of matter (solids, liquids, gases and how they change), separation techniques (how to get pure substances), chemical tests (how to identify unknown substances), and quantitative chemistry (calculating masses, volumes and concentrations). These topics tie together everything from Modules 1–3 and appear heavily on the CSCA exam. Bonding alone can be worth 10–15 marks, and mole calculations regularly appear as 5-mark extended-response questions. By the end of this module you will be able to explain why diamond is hard but graphite is slippery, calculate the mass of product from any reaction, identify mystery gases with simple bench tests, and tackle titration calculations with confidence.
Inorganic chemistry and quantitative analysis make up a significant portion of the CSCA Chemistry paper — expect 30–40% of marks. Bonding questions (compare ionic/covalent/metallic) appear almost every year as 6-mark questions. Mole calculations (mass→moles→mass), titration calculations, and gas volume calculations are guaranteed. Know your chemical tests for gases and ions — these are quick marks if you've memorised the table. Electrolysis half equations are common 3–4 mark questions. Always show your working in calculations — even if your final answer is wrong, you can earn method marks.
Covalent bonding occurs when two non-metal atoms share one or more pairs of electrons so that each atom achieves a full outer shell. The shared pair of electrons is attracted to both nuclei, holding the atoms together.
A single bond shares one pair (2 electrons) — for example H–H in hydrogen (H₂). A double bond shares two pairs (4 electrons) — for example O=O in oxygen (O₂). A triple bond shares three pairs (6 electrons) — for example N≡N in nitrogen (N₂). More shared pairs = shorter, stronger bond.
To draw a dot-and-cross diagram: (1) Work out how many outer-shell electrons each atom has. (2) Put the atoms next to each other. (3) Share electrons in the overlap region so every atom gets a full outer shell (2 for hydrogen, 8 for everything else). Use dots for one atom's electrons and crosses for the other's.
| Molecule | Shared pairs | Bond type | Shape |
|---|---|---|---|
| H₂ | 1 | Single | Linear |
| H₂O | 2 (+ 2 lone pairs on O) | Single ×2 | Bent |
| CO₂ | 4 (2 double bonds) | Double ×2 | Linear |
| CH₄ | 4 (single ×4) | Single ×4 | Tetrahedral |
| N₂ | 3 | Triple | Linear |
Simple covalent substances have low melting/boiling points because the intermolecular forces (forces between molecules) are weak — even though the covalent bonds within each molecule are strong. They do not conduct electricity because there are no free ions or delocalised electrons.
This is a favourite exam question! When ice melts, you are not breaking O–H bonds — you are overcoming the weak forces between H₂O molecules. The covalent bonds inside each water molecule stay intact. This is why water's boiling point (100 °C) is relatively low: the intermolecular forces need only a little energy to overcome.
💡Covalent bonds form when non-metals share electron pairs. Simple covalent molecules have low melting points because weak intermolecular forces (not the strong covalent bonds) break when the substance melts or boils.
📋 Key Formulas
Single bond = 1 shared pair (2 e⁻) | Double bond = 2 shared pairs (4 e⁻) | Triple bond = 3 shared pairs (6 e⁻) | Full shell: H = 2, others = 8 | Simple covalent → low m.p./b.p., no conduction
📝 Worked Example 1
Example 1: Draw the dot-and-cross diagram for water (H₂O).
Step 1: Oxygen has 6 outer electrons; hydrogen has 1 outer electron each.
Step 2: O needs 2 more electrons → shares 1 electron with each H.
Step 3: Each H has 2 electrons (full), O has 8 electrons (full). O also has 2 lone pairs not involved in bonding.
Answer: The diagram shows O in the centre with 2 shared pairs (one with each H) and 2 lone pairs.
📝 Worked Example 2
Example 2: Explain why methane (CH₄) has a low boiling point (−162 °C).
Step 1: CH₄ is a simple covalent molecule.
Step 2: The intermolecular forces between CH₄ molecules are very weak.
Step 3: Only a small amount of energy is needed to overcome these weak forces, so the boiling point is low.
Key: Do NOT say "the covalent bonds are weak" — the C–H bonds are strong; it's the forces between molecules that are weak.
📝 Worked Example 3
Example 3: Why does CO₂ not conduct electricity?
Step 1: CO₂ is a simple covalent molecule.
Step 2: It has no free electrons or ions.
Step 3: Therefore it cannot carry an electric charge, so it does not conduct.
🧠When asked "why does [molecule] have a low boiling point?", always mention: (1) simple molecular structure, (2) weak intermolecular forces, (3) little energy needed to overcome them. Never say "weak covalent bonds"!
🧠Dot-and-cross diagrams: count total outer electrons first. If the total isn't even, you've made an error. For the CSCA exam, always show lone pairs on the central atom.
⚠️Saying "the bonds are weak": The covalent bonds WITHIN molecules are strong. It's the INTERMOLECULAR forces between molecules that are weak. This distinction is worth 1–2 marks on almost every bonding question.
⚠️Forgetting lone pairs: In H₂O, oxygen has 2 lone pairs. In NH₃, nitrogen has 1 lone pair. Always show them.
🎯 Try This Yourself
Nitrogen (N₂) has a boiling point of −196 °C. Explain why, in terms of bonding and structure. [3 marks]
Open and read all sections to complete this module