DSE Chemistry | Chemical Bonding and Structure Lesson Prep (for tutors) | Count the electrons first, then judge the melting point

2026-11-04

Tutor lesson-prep guides · Secondary Chemistry

Students treat any compound containing a metal as ionic, so AlCl3 and CO2 are misclassified. This lesson drills one move: decide whether electrons are transferred or shared, then use the structure to explain melting point and conductivity. Two steps are enough.

Objectives — Students can: (1) draw dot diagrams for ionic and covalent substances from atomic numbers and state the electron arrangement of each ion; (2) use the four structure types, ionic, simple molecular, giant covalent and metallic, to explain melting point and conductivity; (3) compare two melting points by comparing the strength of the forces between particles, not the size of the particles.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Atomic number and arrangement	What is the atomic number of sodium and how many outer electrons?	11 and 1
Periodic table position	Magnesium is in Group II, how many electrons does it lose?	2, forming Mg2+
Classifying substances	Salt water conducts, does sugar water conduct?	No

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three-question check	Electron arrangement, ion charge, conductivity	Answer orally
8-22 min	Ionic bonding	From electron transfer to the lattice	Draw MgO unaided
22-36 min	Covalent and metallic	Shared pairs, sea of electrons	Draw H2O and CH4
36-50 min	Structure decides properties	Four structures against melting point and conductivity	Fill the table
50-57 min	Timed practice	Three questions, easy to hard	Work alone
57-60 min	Exit check	Three checkpoints, one question each	Self-mark and circle slips

03 Teaching script (read it out)

Scene 1: Ionic bonding: transfer first, lattice second

Start from atomic numbers. Sodium has 11 electrons and 1 in the outer shell; losing it leaves 10, the same arrangement as neon, so we write Na+. Chlorine has 17 and gains one to reach 18, the arrangement of argon, so it becomes Cl-. The opposite charges hold the ions in a 6:6 giant lattice, empirical formula NaCl, relative formula mass 58.5. Key line: an ionic compound contains no molecules, only ions in a fixed ratio.

Make students repeat the sentence for MgO: Mg loses 2 to become Mg2+, O gains 2 to become O2-, charges 2+ and 2-.

Scene 2: Covalent and metallic: count the shared pairs

For covalent bonds, count shared pairs and lone pairs. Water has 2 shared pairs and 2 lone pairs on oxygen; ammonia has 3 shared and 1 lone; carbon dioxide has two double bonds, 4 shared pairs in total, and no lone pair left on carbon; methane has 4 shared pairs. For metals use the sea of electrons: outer electrons are delocalised, so metals conduct and are malleable. Ions in an ionic solid are locked by electrostatic attraction, so the solid does not conduct; only the molten or dissolved form carries free ions.

Three habits: count the pairs, mark the lone pairs, and name the freely moving charge before claiming conduction.

Scene 3: Structure decides properties: compare forces, not sizes

The four structures differ hugely in melting point: ionic NaCl at 801 degrees and MgO at 2852; giant covalent diamond at 3550; simple molecular iodine at 114, water at 100 and carbon dioxide subliming at -78; metals from sodium near 98 up to tungsten above three thousand. MgO beats NaCl not because the ions are small but because the 2+ and 2- charges attract far more strongly. Simple molecular substances melt low because only the weak forces between molecules must be overcome, not the covalent bonds inside them.

Every comparison answer has three steps: which structure, which force must be overcome, therefore higher or lower.

04 Question types and worked steps

Type	Example	Steps	Answer
Dot diagrams	Draw NaCl and MgO	Fix the charges, then brackets and electrons	Mg2+ and O2- with eight outer electrons
Covalent structures	Draw H2O and CO2	Count shared and lone pairs, double bonds take two pairs	Each oxygen in CO2 has two lone pairs
Melting point comparison	NaCl against MgO	Same structure type, compare charge and distance	MgO higher, 2,852 against 801
Conductivity	Solid salt and copper	Find the freely moving charge	Copper has delocalised electrons, solid salt has none

Sanity check (teach this): Magnitudes must behave: the relative formula mass of NaCl is 23.0 plus 35.5, which is 58.5, and of MgO is 24.3 plus 16.0, which is 40.3; both are 1:1, so the difference comes from the ions themselves. The melting order must run diamond 3550, then MgO 2852, then NaCl 801, then iodine 114, with dry ice lowest at -78. If a student calls water at 100 degrees a high melting point, they still underestimate how weak the forces between simple molecules are.

05 Independent practice (3 questions)

Question	Answer	What to watch
Draw the dot diagram of MgCl2 with charges	Mg2+ and two Cl-	Is the 2+ balanced by two 1-
Calculate the relative formula masses of NaCl and MgO	58.5 and 40.3	Are relative atomic masses used
Predict conductivity of solid and molten MgO	Solid no, molten yes	Is the free ion mentioned

06 Common mistakes and what to say

Mistake	Student writes	Say this
Any metal means ionic	AlCl3 written as Al3+ and three Cl-	AlCl3 melts low and sublimes easily, so it is simple molecular, not a lattice.
Thinking covalent bonds break on melting	Water melts high because O-H bonds are strong	Melting ice only overcomes forces between molecules; the O-H bonds stay.
Conduction answered as having electrons	Solid NaCl conducts because it has electrons	Name the freely moving charge; in a solid the ions are locked.
Comparing melting points by size only	MgO lower than NaCl because magnesium is small	For the same lattice type compare charge and distance; 2+ against 2- wins.

07 Homework (paper, section and question numbers)

Chemistry subject index — All chapters
Chemistry practice paper — Chemistry section: dot diagrams and melting point comparison

08 Exit check (last 5 minutes)

Question	Answer	Decision
Ion charges	What ions do Mg and O form	Cannot say: reteach act one
Structure type	Which type is CO2	Says giant: back to act two
Conductivity reason	Why molten NaCl conducts	Only electrons: add freely moving ions

09 Tutor FAQ

Why does diamond melt higher than any ionic compound?

It is giant covalent, so melting means breaking very many strong covalent bonds, which costs a great deal of energy.

Graphite conducts, is that an exception?

No. Each layer has delocalised electrons, so it conducts, while the weak forces between layers still leave a very high melting point.

Must students memorise the numbers?

Only the order of magnitude: MgO above NaCl, simple molecular lowest. Exact values are given in questions.

Find a tutor — free Tutor Registration →

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.