AQA A-Level Chemistry Paper 3, June 2025: Question 20

1 mark · Easy difficulty · Multiple Choice

Identify which property of the Group 2 elements increases with increasing atomic number.

Practise this question

Question

Multiple choice question asking: 'Which property of the Group 2 elements increases with increasing atomic number?' with four options: A Atomic radius, B Electronegativity, C First ionisation energy, and D Melting point.
Question text

20 Which property of the Group 2 elements increases with increasing atomic number?

[1 mark]

A Atomic radius

B Electronegativity

C First ionisation energy

D Melting point

Mark scheme

Show the mark scheme Mark scheme table indicating Question 20 has the correct answer A, worth 1 mark (AO1), corresponding to Atomic radius.

20 A 1 (AO1) Atomic radius

How to answer it

Group 2 Periodic Trends: Properties Down the Group

📋 What this question tests

This question assesses knowledge of fundamental periodic trends in Group 2 (the alkaline earth metals) as atomic number increases down the group. Specifically, it tests your understanding of trends in atomic radius, first ionisation energy, electronegativity, and melting point, and the underlying atomic factors (electron shielding, nuclear charge, and distance from nucleus) that drive them.

Question 20 (Multiple Choice)

Identifying the property that increases down Group 2

Mark scheme: A — 1 mark (AO1)

✅ Correct Answer: Option A

A: Atomic radius

As you descend Group 2 (increasing atomic number from Be to Ba), each successive element has one additional principal electron energy level (electron shell). This extra shell increases the physical distance between the outer electrons and the nucleus, causing atomic radius to increase.

💡 Key Knowledge: Why the Other Options Decrease

  • Electronegativity (Option B): Decreases down the group. The bonding pair of electrons is further from the nucleus and shielded by more inner shells, reducing electrostatic attraction.
  • First Ionisation Energy (Option C): Decreases down the group. Increased shielding and greater atomic radius outweigh the increase in nuclear charge, so the outermost electron is less tightly held and more easily lost.
  • Melting Point (Option D): Generally decreases down the group. The metallic cations (M²⁺) get larger, so the delocalised sea of electrons is further from the positive nuclei, resulting in weaker metallic bonding. (Note: Mg has an anomalous dip due to a different crystal packing structure, but the overall trend down the group is a decrease).

🧠 Exam Technique: "Increasing Atomic Number"

  • Decode the question phrase: "With increasing atomic number" in a named group question simply means going down the group (from Be to Mg to Ca to Sr to Ba).
  • Recall the 'Big Rule': Down any group, atomic radius always increases because additional quantum shells are added.
  • Rapid elimination: First ionisation energy and electronegativity are closely linked properties that almost universally decrease down any main group. You can immediately rule out B and C.

❌ Common Errors & Traps

  • Nuclear Charge Trap: Thinking that because proton number increases, nuclear attraction must increase and pull outer electrons closer. Students forget that extra shielding and distance completely outweigh nuclear charge down a group.
  • Reactivity vs. Ionisation Energy Confusion: Reactivity increases down Group 2 because ionisation energy decreases. Do not mix up the trend in reactivity (increases) with ionisation energy (decreases).
  • Melting Point Assumption: Assuming heavier metal atoms mean a higher melting point. Metallic bonding strength actually weakens as the cationic radius increases.

📐 Summary of Group 2 Trends (Be → Ba)

Property Trend down Group 2 Key Reason
Atomic Radius Increases More occupied electron shells.
First Ionisation Energy Decreases Increased radius + shielding outweigh increased nuclear charge.
Electronegativity Decreases Bonding pair further from nucleus and more shielded.
Melting Point Decreases (overall) Larger M²⁺ ionic radius leads to weaker electrostatic attraction to delocalised electrons.
Reactivity with H₂O Increases Easier to lose two valence electrons (lower sum of 1st + 2nd IE).

Topics

Inorganic Chemistry · 3.2.2 Group 2, The Alkaline Earth Metals

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.