Period 3 Elements: Melting Point and Conductivity
Explain physical-property patterns using structure and bonding.
On this page
Period 3 “melting point + conductivity” questions are bonding questions in disguise. Your marks come from one chain: structure type → bonding/forces → property.
Interpret these trends using electron configurations from Atomic Structure, then use your course’s Periodic Table topic navigation to compare groups and periods.
Definitions (Must Know)
A. Metallic bonding
Metallic bonding is the electrostatic attraction between a lattice of positive ions and delocalised electrons.
B. Giant covalent structure
A giant covalent structure is a network of atoms linked by covalent bonds throughout the structure.
C. Simple molecular substance
A simple molecular substance consists of molecules held together by intermolecular forces.
D. Electrical conductivity
Electrical conductivity is the ability to conduct electricity due to mobile charge carriers (mobile ions or electrons).
E. Delocalised electrons
Delocalised electrons are electrons that are free to move through a structure (not held in one bond).
Key Ideas (What Earns Marks)
- Across Period 3 (Na → Cl), the structure changes, so the melting point and conductivity change.
- Structure across Period 3:
- Na, Mg, Al: metallic lattices (delocalised electrons)
- Si: giant covalent (network)
- P₄, S₈, Cl₂: simple molecular (London forces)
- Conductivity:
- metals conduct as solids (delocalised electrons)
- simple molecular substances do not conduct (no mobile charge carriers)
- silicon is a semiconductor (few mobile electrons at room temperature)
“It has a [structure type], so the particles are held by [bonding/forces]. Therefore [property] is high/low because [why].”
Detailed Explanations
A. Structure across Period 3 (what to state first)
| Element (Period 3) | Structure type (solid) | What holds it together |
|---|---|---|
| Na, Mg, Al | metallic lattice | metallic bonding (ions + delocalised electrons) |
| Si | giant covalent | covalent bonds throughout a network |
| P, S, Cl | simple molecular | intermolecular forces (mainly London forces) |
B. Melting point pattern (how to explain the “shape”)
Typical pattern across Period 3:
- Na → Mg → Al: melting point increases (stronger metallic bonding).
- Si: very high melting point (many strong covalent bonds throughout the network).
- P → S → Cl: low melting points (simple molecules held by London forces), but larger molecules have stronger London forces.
Data table
| Element | mp |
|---|---|
| Na | 98 |
| Mg | 650 |
| Al | 660 |
| Si | 1410 |
| P | 44 |
| S | 115 |
| Cl | -101 |
C. Why metallic bonding strengthens from Na to Al
Across Na → Mg → Al:
- cation charge increases (+ 1 → + 2 → + 3)
- cation size decreases (higher Z, same shell) → higher charge density
- number of delocalised electrons per atom increases (1 → 2 → 3)
So the electrostatic attraction between positive ions and delocalised electrons becomes stronger → higher melting point.
D. Conductivity pattern (what to say)
- Metals (Na, Mg, Al): conduct as solids because delocalised electrons are mobile.
- Silicon: does not have delocalised electrons like a metal; it is a semiconductor (conducts poorly at room temperature).
- Simple molecular solids (P₄, S₈, Cl₂): do not conduct because there are no mobile ions or electrons.
Worked Examples
Modelled example 1
Explain the Sodium-to-Aluminium Melting Trend
Problem
Study the worked solution
Hold the structure type constant
Method
Classify sodium, magnesium and aluminium as giant metallic lattices.Reason
The comparison is between strengths of metallic bonding, not different structure families.Working
Positive metal ions in a sea of delocalised electrons.Compare charge density and electrons
Method
Increase cation charge, decrease cation radius and increase the number of delocalised electrons per atom.Reason
These changes strengthen electrostatic attraction between the ions and delocalised electrons.Working
Na⁺ → Mg²⁺ → Al³⁺ with increasing charge density.Link to melting
Method
Conclude that more energy is required to overcome the stronger metallic bonding.Reason
Melting point tracks the energy needed to disrupt the lattice.Working
Tₘ(Na) < Tₘ(Mg) < Tₘ(Al).
Guided practice 2
Compare Silicon and Aluminium
Problem
Explain why silicon has a much higher melting point than aluminium.
Try this before viewing the solution
Hints
Hint 1: structure
Name the structure of each element before comparing bond strengths.
Hint 2: energy
State what must be overcome when each solid melts.
View solution step by step
Classify
Method
Describe silicon as giant covalent and aluminium as metallic.
Reason
The sharp change arises from a change in structure and bonding.
Working
Si: covalent network; Al: metallic lattice.
Compare energy
Method
State that melting silicon requires breaking many strong covalent bonds throughout its network.
Reason
This requires substantially more energy than weakening metallic bonding in aluminium.
Working
Tₘ(Si)≫ Tₘ(Al).
Common misconception 3
Compare Sulfur and Chlorine Molecular Solids
Learner claim
A learner says sulfur melts above chlorine because the covalent bonds inside S₈ are stronger than those inside Cl₂. Correct the explanation.
Try this before viewing the solution
View solution step by step
Identify the relevant force
Method
Compare London forces between molecules.Reason
Both substances are simple molecular and retain their covalent molecules on melting.
Working
S₈ molecules and Cl₂ molecules.
Compare polarisability
Method
Use the larger electron cloud of S₈.Reason
Its greater polarisability produces stronger London forces and requires more energy to overcome.
Working
Tₘ(S₈) > Tₘ(Cl₂).
Challenge 4
Identify a Period 3 Element from Physical Data
Data transfer
Try this before viewing the solution
Hints
Hint 1: conductivity
Hint 2: melting point
View solution step by step
Infer metallic structure
Method
Use mobile delocalised electrons to identify a Period 3 metal.Reason
Solid-state conduction rules out the simple molecular non-metals.Working
Candidates: Na, Mg or Al.Use the melting datum
Method
Select sodium.Reason
Its melting point is close to 98°C, whereas magnesium and aluminium melt near 650°C.Working
Na.
Common Mistakes
- Explaining melting points without naming the structure type first.
- Saying “strong covalent bonds” for P₄, S₈, Cl₂ melting points (melting separates molecules, so it overcomes intermolecular forces).
- Calling silicon “simple molecular” (it is giant covalent).
- Conductivity answers that don’t name the charge carriers (delocalised electrons vs mobile ions).
Use the topic check in Practise and check below to practise and check your understanding.
Exam Tips
- Use the two keywords early: metallic lattice / giant covalent / simple molecular.
- If comparing P vs S vs Cl, your driver is usually London forces (more electrons / larger molecule → stronger London forces).
- Link every claim to the property asked: “therefore higher melting point” / “therefore does not conduct”.
Mind Stretchers
Mind stretcher 1Extension
Explain why the melting point rises sharply from Al to Si, but then drops sharply from Si to P.
Show Answer
Mark scheme:
- Si is giant covalent, so melting requires breaking many covalent bonds throughout a network → very high melting point.
- P exists as simple molecular P₄, so melting only overcomes intermolecular forces between molecules → much lower melting point.
- Therefore the change in structure (giant covalent → simple molecular) explains the sharp drop.
Mind stretcher 2Extension
A Period 3 element forms an oxide that is acidic and a chloride that hydrolyses to produce steamy fumes. Which region of Period 3 is it likely to come from, and why?
Show Answer
Mark scheme:
- Acidic oxides and hydrolysing covalent chlorides are typical of non-metals to the right of Period 3.
- Across Period 3, bonding becomes more covalent; covalent oxides are acidic and covalent chlorides hydrolyse to produce HCl fumes.
- So it likely comes from the right-hand side of Period 3 (Si, P, S, Cl region).
Syllabus and review details
- GCE A-Level H2 Chemistry 9476-2027 · 9476-2027
9476 (2027), complete syllabus
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