DSE Chemistry | Bonding and Structure Lesson-Prep Guide (Tutor Edition) | Melting Point, Conductivity and Lattice Energy

2026-11-04

Tutor lesson-prep guides · Secondary Chemistry

Students can chant that metal plus non-metal means ionic, but go silent when asked why magnesium oxide is harder to melt than sodium chloride. This lesson gives them two variables: charge product and distance.

Objectives — Students can: (1) decide the bond type from position in the periodic table and electron transfer; (2) compare lattice energy using charge product over distance and infer melting point; (3) explain conductivity, malleability and hardness from structure.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Ion charges	Charges on magnesium and oxygen in MgO	Mg2+ and O2-
Unit conversion	How many pm in 1 nm	1000 pm
Electron arrangement	Sodium after losing one electron	2,8

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Four structures	Ionic, simple molecular, giant covalent, metallic	Two examples each
8-20 min	Bond type	Metal plus non-metal, transfer or share	Classify six compounds
20-34 min	Melting point	Charge product divided by distance	Get about 5.0 times
34-46 min	Giant structures	Diamond, graphite, silicon dioxide	Explain graphite conduction
46-54 min	Conduction and bending	Delocalised electrons and mobile ions	Rewrite one answer
54-60 min	Exit check	Three questions marked on the spot	Independent work

03 Teaching script (read it out)

Scene 1: Four structures in one table

Ionic: metal cations and non-metal anions held in a three-dimensional lattice by electrostatic attraction; hard and brittle, and conducting only when molten or dissolved. Simple molecular: covalent bonds inside molecules but only weak van der Waals forces between them, so low melting and boiling points, as in water and carbon dioxide. Giant covalent: the whole solid is one covalent network, so melting points are very high, as in diamond, graphite and silicon dioxide. Metallic: cations in a sea of delocalised electrons, giving conduction, heat transfer and malleability.

Tutor move: draw four boxes on the board, put one example in each, then make every later question start with which box it belongs to.

Scene 2: Melting point: charge product over distance

Sodium chloride: Na+ is 102 pm and Cl- is 181 pm, so the ion distance is about 283 pm and the charge product is 1 times 1. Magnesium oxide: Mg2+ is 86 pm and O2- is 140 pm, so the distance is about 226 pm and the charge product is 2 times 2, which is 4. The relative lattice energy is therefore 4 times 283 divided by 226, about 5.0 times, which is why the melting point rises from 801 C to 2852 C.

Ask the student: both ions carry charge 2 and the distance halves, so how many times larger is the lattice energy (4 divided by 0.5, that is 8 times).

Scene 3: Giant covalent and metallic structures

Diamond: each carbon joins four others with 154 pm covalent bonds in a three-dimensional network, so it is extremely hard and does not conduct. Graphite: bonds of 154 pm within a layer, but layers are 335 pm apart with only van der Waals forces, so layers slide and the solid feels soft, while the fourth electron on each carbon is delocalised along the layer and conducts. Silicon dioxide: a giant covalent network melting at 1710 C, which is why it is used as sand and glass. Metals: delocalised electrons move freely, so they conduct, and cation layers slide without breaking, so they are malleable.

Warn: graphite conducts because of delocalised electrons within the layer, not because of free ions.

04 Question types and worked steps

Type	Example	Steps	Answer
Identify	Bond type and structure from a formula	Check element position, then whether it is giant	NaCl ionic, CO2 simple molecular
Compare	Rank melting points	Structure type first, then charge and distance	MgO at 2852 C above NaCl at 801 C
Explain	Why solid sodium chloride does not conduct	Ions are locked in the lattice	Conducts when molten or dissolved
Draw	Dot-and-cross diagrams	Count outer electrons, then pair them	MgO needs 2+ and 2-

Sanity check (teach this): Structure type comes first: giant structures always melt high (silicon dioxide at 1710 C) and simple molecular ones always melt low. Only among ionic solids do you compare charge and distance, and lattice energy falls as distance rises, so bigger ions mean lower melting points.

05 Independent practice (3 questions)

Question	Answer	What to watch
Ion distance in sodium chloride and magnesium oxide	283 pm and 226 pm	Add the radii, never subtract
Lattice energy of MgO relative to NaCl	about 5.0 times	Charge product 4, then distance ratio
Why graphite conducts but diamond does not	Delocalised electrons within each layer	Say within the layer, not between layers

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating graphite as simple molecular	Graphite melts low because the layers are weakly held	“Weak between layers means soft; melting needs the strong bonds inside the layer to break.”
Using charge only for melting point	MgO melts high only because the charges are bigger	“Divide the charge product by the distance, that is the lattice energy.”
Claiming solid ionic solids conduct	Solid salt conducts electricity	“Ions are locked in the lattice; they only move when molten or dissolved.”
Explaining metal bending as ion sliding alone	Metal ions slide so the metal does not break	“Metal has a sea of electrons, not ionic bonds.”

07 Homework (paper, section and question numbers)

DSE chemistry practice paper (structure and properties) — Section A identification and explanation
Secondary chemistry chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Which melts higher, MgO or NaCl	MgO, at 2852 C	All three right: move to acids and bases
Bond length within diamond and graphite layers	Both about 154 pm	One wrong: reteach structure
Does solid sodium chloride conduct	No	Two wrong: restart act one

09 Tutor FAQ

Must the ionic radii be memorised?

No, but students should know radius grows down a group, so melting point falls.

Why does silicon dioxide melt higher than sodium chloride?

Breaking a giant covalent network costs far more than pulling apart an ionic lattice.

Why does water boil higher than hydrogen sulfide?

Water has hydrogen bonds, hydrogen sulfide only van der Waals forces, hence 100 C against -60 C.

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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.