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Period 3 Elements: Melting Point and Conductivity

Learn and apply Period 3 Elements: Melting Point and Conductivity in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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Period 3 Elements: Melting Point and Conductivity: Orientation

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 with the Atomic Structure hub beside you, then use the Periodic Table (A Level) hub 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).

Detailed Explanations

A. Structure across Period 3 (what to state first)

Element (Period 3)Structure type (solid)What holds it together
Na, Mg, Almetallic latticemetallic bonding (ions + delocalised electrons)
Sigiant covalentcovalent bonds throughout a network
P, S, Clsimple molecularintermolecular forces (mainly London forces)
Period 3 structure-to-property mapThree panels comparing metallic, giant covalent, and simple molecular structures in Period 3 with one-line property links for melting point and electrical conductivity.Period 3: structure first, then explain propertyNa, Mg, Al: metallic latticeStrong electrostatic attractionin latticeMelting point: rises Na -> Mg ->AlConductivity: high (mobileelectrons)Si: giant covalent networkMany strong covalent bonds in 3DnetworkMelting point: very high (peakin Period 3)Conductivity: low at room temp(semiconductor)P4, S8, Cl2: simple molecularDashed lines = weak London forcesMelting overcomes intermolecularforcesLow m.p.; S₈ higher than P₄ andCl₂No conduction: no mobile charges
Period 3 structure map: metallic lattices (Na to Al), giant covalent Si, and simple molecular non-metals (P4, S8, Cl2), each linked directly to melting point and conductivity.

B. Melting point pattern (how to explain the “shape”)

Typical pattern across Period 3:

  1. Na → Mg → Al: melting point increases (stronger metallic bonding).
  2. Si: very high melting point (many strong covalent bonds throughout the network).
  3. P → S → Cl: low melting points (simple molecules held by London forces), but larger molecules have stronger London forces.
Period 3 Melting Points (Approximate)Representative values to show the exam pattern: metals increase (Na→Al), Si is very high (giant covalent), then molecular solids are low (P, S, Cl).Period 3 Melting Points (Approximate)ElementMelting point (°C)
Representative values to show the exam pattern: metals increase (Na→Al), Si is very high (giant covalent), then molecular solids are low (P, S, Cl).
Data table
Elementmp
Na98
Mg650
Al660
Si1410
P44
S115
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

Core

Problem

Explain why melting point increases from sodium to aluminium across Period 3.
Study the worked solution

  1. 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.

  2. 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.

  3. 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

About 5 min

Problem

Explain why silicon has a much higher melting point than aluminium.

Try this before viewing the solution

Silicon structure
During silicon melting

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

  1. 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.

  2. 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

Find and correct the mistake

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

Force overcome on melting

View solution step by step

  1. 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.

  2. 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

Minimal support

Data transfer

A Period 3 element conducts electricity well as a solid and melts close to 100 °C. Suggest the element and justify your choice using structure and bonding.

Try this before viewing the solution

Structure implied by solid conduction
Element near 100°C

Hints

Hint 1: conductivity
Use the mobile charge carrier to narrow the structure family.
Hint 2: melting point
Distinguish sodium from magnesium and aluminium using the supplied temperature.
View solution step by step

  1. 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.

  2. 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.

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).