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DSE Chemistry | Chemical Bonding and Structure: Lesson-Prep Guide for Tutors | Ionic, Covalent and Melting Points
Students learn “metal plus non-metal means ionic” and then call aluminium chloride ionic as well. This lesson teaches one decision order: read the elements, then the structure, and only then the properties.
Objectives — Students can: (1) decide the bond type from element position and draw the transfer or shared pair; (2) convert mass to moles and moles to concentration; (3) use structure to explain melting point and conductivity.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Molar mass | Relative formula mass of NaCl | 58.5 |
| Moles from mass | How many moles in 11.7 g of NaCl | 0.2 mol |
| Concentration | Moles divided by volume in cubic decimetres | mol per cubic decimetre |
02 Lesson flow (60 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-10 min | Electron transfer | Metals lose, non-metals gain | Write Na+ and Cl- |
| 10-22 min | Ionic bonding | Giant ionic lattice, electrostatic attraction | Sketch the lattice |
| 22-34 min | Covalent bonding | Shared pairs, single double triple bonds | Dot diagrams of water and carbon dioxide |
| 34-46 min | Four structures | Giant ionic, simple molecular, giant covalent, metallic | Sort six substances |
| 46-56 min | Properties and arithmetic | Melting point, conductivity, concentration | 0.8 mol per cubic decimetre |
| 56-60 min | Exit check | Three questions marked on the spot | Done independently |
03 Teaching script (read it out)
Scene 1: Count electrons before drawing bonds
A sodium atom has one outer electron and a chlorine atom is one short of a full shell, so sodium hands the electron over and the pair becomes Na+ and Cl-. The ions attract electrostatically into a giant ionic lattice, written NaCl.
Two non-metals behave in the opposite way: neither will give an electron away, so they share a pair. Water has two single bonds, carbon dioxide two double bonds, nitrogen one triple bond.
Two non-metals behave in the opposite way: neither will give an electron away, so they share a pair. Water has two single bonds, carbon dioxide two double bonds, nitrogen one triple bond.
Ask the student to draw the transfer in MgCl2: magnesium loses two electrons and each chlorine gains one.
Scene 2: Four structures, and every property follows from them
Giant ionic (NaCl): ions packed tightly, and breaking it up means breaking many electrostatic attractions, so the melting point is high (about 801 degrees). The solid does not conduct, but the melt or the solution does.
Simple molecular (carbon dioxide, water): only weak intermolecular forces between molecules, so melting and boiling points are low and there is no conduction. Water molecules hydrogen bond, up to 4 hydrogen bonds per molecule.
Giant covalent (diamond, quartz): one continuous covalent network, extremely high melting point, no conduction, with graphite the exception because of its delocalised electrons.
Metallic: a sea of delocalised electrons, conducts heat and electricity, and is malleable.
Simple molecular (carbon dioxide, water): only weak intermolecular forces between molecules, so melting and boiling points are low and there is no conduction. Water molecules hydrogen bond, up to 4 hydrogen bonds per molecule.
Giant covalent (diamond, quartz): one continuous covalent network, extremely high melting point, no conduction, with graphite the exception because of its delocalised electrons.
Metallic: a sea of delocalised electrons, conducts heat and electricity, and is malleable.
Reminder: the answer to “why does NaCl melt so high” is that many electrostatic attractions must be broken, never “strong covalent bonds”.
Scene 3: Concentration, and the two traps
Concentration is moles divided by volume in cubic decimetres. 11.7 g of NaCl is 11.7 / 58.5 = 0.2 mol; dissolved in 250 cubic centimetres, which is 0.25 cubic decimetres, the concentration is 0.2 / 0.25 = 0.8 mol per cubic decimetre.
Cubic centimetres must be converted first by dividing by 1000; dividing by 250 directly is the single most common error.
Aluminium chloride has substantial covalent character, melts near 190 degrees and sublimes easily, and is the standard school-level exception to the metal-plus-non-metal rule.
Cubic centimetres must be converted first by dividing by 1000; dividing by 250 directly is the single most common error.
Aluminium chloride has substantial covalent character, melts near 190 degrees and sublimes easily, and is the standard school-level exception to the metal-plus-non-metal rule.
Ask the student to compute: the same 0.2 mol in 500 cubic centimetres gives 0.4 mol per cubic decimetre.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Draw | Electron transfer in NaCl | Lose and gain, label the charges | Na+ and Cl- |
| Classify | Sort six substances by structure | Elements first, then arrangement | Giant, simple, metallic |
| Explain | Compare melting points of NaCl and CO2 | Name the force being broken | Electrostatic versus intermolecular |
| Calculate | 11.7 g NaCl in 250 cubic centimetres | Moles over cubic decimetres | 0.8 mol per cubic decimetre |
Sanity check (teach this): A school concentration usually sits between 0.1 and 2 mol per cubic decimetre, so 80 or 0.0008 means the division by 1000 was missed. The mole value must be smaller than the mass because the formula mass exceeds 1: 11.7 g gives 0.2 mol. When comparing melting points, compare within one structure type, or state which force has to be broken.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| Relative formula mass of NaCl | 58.5 | 23 plus 35.5 |
| Moles in 11.7 g of NaCl | 0.2 mol | Mass divided by formula mass |
| Concentration in 250 cubic centimetres | 0.8 mol per cubic decimetre | Convert to cubic decimetres first |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Cubic centimetres treated as cubic decimetres | Answers 0.0008 | 250 cubic centimetres is 0.25 cubic decimetres. |
| Metal plus non-metal always ionic | Calls aluminium chloride ionic | Check the real properties; its melting point is low. |
| Using covalent bonds to explain NaCl | Says strong covalent bonds raise the melting point | NaCl has no covalent bonds, only electrostatic attraction. |
| Intermolecular force confused with a bond | Says boiling water breaks the O-H bond | Boiling breaks hydrogen bonds, not O-H bonds. |
07 Homework (paper, section and question numbers)
- Secondary Chemistry chapter index — All chapters
- DSE Chemistry practice paper (structure and properties) — Section A bonding questions
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Moles in 11.7 g of NaCl | 0.2 mol | All three correct: move to acids |
| Concentration in 250 cubic centimetres | 0.8 mol per cubic decimetre | One wrong: reteach units |
| Why carbon dioxide melts low | Weak intermolecular forces | Two wrong: restart scene two |
09 Tutor FAQ
Why does solid ionic compound not conduct?
The ions are locked in the lattice and cannot move; melting or dissolving frees them.
Why does graphite conduct but diamond not?
Graphite has delocalised electrons between layers; in diamond every electron sits in a covalent bond.
Is a hydrogen bond a chemical bond?
No. It is a strong intermolecular force, far weaker than a covalent bond.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.
