Edexcel A-Level Chemistry Paper 1, June 2023: Question 4

6 marks · Medium difficulty · Extended Writing

Explain the variations in melting temperature across Period 3 elements (Na, Mg, Al, Si, P) in terms of structure and bonding.

Practise this question

Question

A line graph showing the melting temperatures in Kelvin on the y-axis (from 0 to 1800) against Period 3 elements (Na, Mg, Al, Si, P) on the x-axis. Points are plotted at roughly 371 K for Na, 923 K for Mg, 933 K for Al, 1683 K for Si, and 317 K for P. Below the graph, the question asks to explain the variations in melting temperature across the period in terms of structure and bonding.
Question text

4 The graph shows the melting temperatures of some elements in Period 3.

1800

1600

1400

1200

Melting

1000

temperature

/ K 800

Na Mg Al Si P

Elements in Period 3

Explain the variations in melting temperature across the period in terms of the

structure and bonding in these elements.

(6)

… *P71912A0628*

(Total for Question 4 = 6 marks)

Mark scheme

Show the mark scheme A mark scheme table detailing six marking points corresponding to metallic bonding trends in Na, Mg, Al, giant covalent bonding in Si, and simple molecular structure with weak London forces in P.

Question

Answer Additional guidance Mark

Number

4 An explanation that makes reference the following Allow marks for any relevant annotations on the graph (6)

trends: Ignore reference to boiling point

Ignore general trends across the period

Na, Mg and Al

• Na, Mg and Al have metallic bonding (in a giant Allow description of metallic bonding – attraction between cations and

lattice) (1) delocalised electrons

Ignore aluminium missing from answer

Do not award simple molecular / London forces / ionic for any of these metals

• charge on ion increases / (cation) radius Allow increasing number / more delocalised electrons

decreases / charge density increases (so strength Allow atomic radius decreases

of metallic bonding increases and melting Ignore nuclear charge / number of protons increases

temperature increases) (1) Ignore any explanation about why Al has similar melting temperature to Mg

Si

• Si is giant covalent (lattice) / giant molecular / Do not award silicon has metallic bonding / London forces

macromolecular (1)

• so a lot of energy required to break (many Allow a lot of energy is needed to overcome the electrostatic forces (in the

strong) covalent bonds (and it has the highest covalent bonds)

melting temperature) (1) Ignore covalent bonding is stronger than metallic bonding

P

• (large decrease from Si to) phosphorus as it Allow P4 molecules

consists of simple / discrete molecules / is Ignore formula of phosphorus even if incorrect

simple molecular (1) Do not award phosphorus has ionic / metallic bonding / permanent dipoles /

hydrogen bonding

• so only weak London forces (are broken on Allow London forces are weaker than covalent bonds / covalent bonds are

melting) (1) stronger than London forces

Allow not a lot of energy is required to overcome the (London) forces

Allow just ‘weak intermolecular forces’

Do not award covalent bonds broken in P

Allow dispersion / temporary dipole-induced dipole / instantaneous dipole / van

der Waals forces as alternatives to London forces throughout

Ignore abbreviations e.g. LDF / VDW / ID-ID

(Total Question 4 = 6 marks)

How to answer it

Period 3 Melting Temperatures Analysis

📌 What this question tests

This 6-mark extended response question assesses your knowledge of periodic trends, bonding types, and crystal structures across Period 3 elements (Na to P). You must link macroscopic physical properties (melting temperature) to microscopic structure and bonding (metallic, giant covalent, and simple molecular).

Question Breakdown

Explain the variations in melting temperature across the period in terms of the structure and bonding in these elements. (6 marks)

💡 Key Knowledge: Section by Section

  • Sodium, Magnesium, Aluminium (Na to Al): Metallic bonding in a giant metallic lattice. Melting point increases due to higher ionic charge, smaller cation radius, and increased delocalised electrons per ion.
  • Silicon (Si): Giant covalent lattice. Has the highest melting point because a vast network of strong covalent bonds requires massive energy to break.
  • Phosphorus (P): Simple molecular structure (e.g., P₄). Sharp drop in melting point because only weak London forces exist between molecules, requiring little energy to overcome.

✅ Examiner-Approved Phrasing

  • "Na, Mg, and Al have metallic bonding consisting of positive ions and delocalised electrons."
  • "The charge on the ion increases and ionic radius decreases, increasing the attraction."
  • "Silicon is giant covalent with many strong covalent bonds requiring a lot of energy to break."
  • "Phosphorus is simple molecular and has weak London forces that are broken on melting."

🧠 Exam Technique

  • Structure your answer logically: Go element by element or group by group (Metals -> Giant Covalent -> Simple Molecular).
  • Link bonding to energy: Always state *what* type of particle/bond is being overcome and relate it directly to the amount of energy required.
  • Check your terminology: Ensure you match the correct force/bond type to the correct structure (never mix up covalent bonds with intermolecular forces).

❌ Common Errors & Pitfalls

  • The "Molecular Metals" Trap: Falsely claiming silicon or metals have London forces or simple molecular structures.
  • Bond Breaking Confusion: Stating that covalent bonds are broken when phosphorus melts (only weak London forces break).
  • Vague Explanations: Saying melting point increases "because atomic number increases" without mentioning charge density or number of delocalised electrons.
Mark Scheme Allocation (6 Marks Total):
• Mark 1: Na, Mg, Al have metallic bonding in a giant lattice.
• Mark 2: Increasing charge / decreasing radius / increasing charge density (stronger metallic bonding / higher melting temp).
• Mark 3: Silicon is giant covalent (lattice / macromolecular).
• Mark 4: A lot of energy required to break many strong covalent bonds.
• Mark 5: Phosphorus consists of simple / discrete molecules.
• Mark 6: Only weak London forces are broken on melting.

Topics

Physical Chemistry · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.