Edexcel A-Level Chemistry AS Paper 1, November 2020: Question 7

9 marks · Hard difficulty · Extended Writing

Define periodicity and illustrate it with atomic radii, then explain the trend in melting temperatures of Period 2 elements in terms of structure and bonding.

Practise this question

Question

Question 7 consists of two parts. Part (a) asks to define periodicity and illustrate the answer by referring to atomic radii of Period 2 and Period 3 elements, worth 3 marks. Part (b) provides a table of melting temperatures for Period 2 elements (Li to Ne) and asks to explain the trend across the elements in terms of their structure and bonding, worth 6 marks.
Question text

7 (a) Give the meaning of the term ‘periodicity’.

Illustrate your answer by referring to the atomic radii of the Period 2 and Period 3 elements.

Specific values of atomic radii are not required.

(3)

*(b) The melting temperatures of the Period 2 elements are shown.

Symbol of the element Li Be B C(diamond) N O F Ne

Melting temperature / K 454 1551 2573 3970 63 55 53 25

Explain the trend in melting temperatures across the elements of Period 2 in

terms of their structure and bonding.

(6)

… 18

… *P62306A01824*

(Total for Question 7 = 9 marks)

Mark scheme

Show the mark scheme The mark scheme for question 7(a) awards marks for defining periodicity as repeating patterns in properties, stating that atomic radii decrease across a period, and noting the pattern repeats in Period 3. For question 7(b), marking points include identifying metallic bonding for Li/Be, increasing metallic bond strength, giant covalent structures for boron/carbon, strong covalent bonds requiring high energy to break, simple molecular structures for N to Ne, and weak London forces between molecules, with additional marks awarded for structure and logical reasoning.

How to answer it

Periodicity and Melting Temperatures Study Guide

What this question tests

This question assesses your understanding of periodicity (repeating trends across periods in the Periodic Table) and your ability to link physical properties (like atomic radius and melting temperature) to underlying structure and bonding (metallic, giant covalent, and simple molecular) across Period 2.

Question Part (a) — Definition & Illustration

Defining Periodicity and Atomic Radius Trends

✅ Correct Answer Requirements

  • Point 1: A trend or pattern of repeating physical and chemical properties with increasing atomic number.
  • Point 2: Atomic radii decrease from left to right across a period.
  • Point 3: The pattern/trend is repeated in Period 3.

💡 Key Knowledge

Periodicity relies on electron configuration. As you move across a period, electrons fill the same principal quantum shell, but nuclear charge increases, pulling the outer electrons closer and reducing atomic radius.

❌ Common Errors

  • Stating a "trend in a group" instead of a period.
  • Forgetting to mention that the pattern repeats in subsequent periods.
Mark Allocation: 3 marks total (1 mark for defining periodicity, 1 mark for atomic radius trend across a period, 1 mark for stating it repeats in Period 3).
Question Part (b) — Extended Response

Explaining Period 2 Melting Temperatures

✅ Indicative Points (IPs) for Full Marks

  • IP1 (Metallic start): Li and Be have metallic bonding.
  • IP2 (Stronger metals): Metallic bonding gets stronger as the number of delocalized electrons increases or cation charge/size increases.
  • IP3 (Giant covalent middle): B and C (diamond/graphite) have giant covalent structures.
  • IP4 (Covalent bond strength): A lot of energy is needed to break strong covalent bonds.
  • IP5 (Simple molecules on RHS): N, O, F, and Ne consist of simple discrete molecules (or individual atoms for Ne).
  • IP6 (Weak intermolecular forces): Weak London forces between molecules require little energy to overcome.

🧠 Exam Technique & Quality of Written Communication (QWC)

This is a 6-mark extended-response question assessed via a levels-of-response mark scheme. To secure top marks (Level 2 for reasoning):

  • Group your points logically: metals first, giant covalent in the middle, simple molecules at the end.
  • Explicitly link the type of structure and bonding to the energy required to melt the substance.

❌ Common Pitfalls

  • Confusing covalent bonds within molecules with intermolecular forces (London forces) between molecules.
  • Incorrectly attributing high melting temperatures to breaking covalent bonds in simple molecular substances like nitrogen ( N₂ ).
Mark Allocation: 6 marks total (Up to 4 marks for Indicative Content based on how many IPs are hit, plus up to 2 marks for structure, logic, and sustained lines of reasoning).

Topics

Physical Chemistry · Inorganic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.