AQA A-Level Chemistry Paper 1, June 2025: Question 2

8 marks · Medium difficulty · State/Explain/Numerical

Explain trends in first and successive ionisation energies, write equations for second ionisation energy, predict the lowest second ionisation energy in Period 3, and determine an ionisation energy value using a logarithmic graph.

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Question

Question text

02 This question is about ionisation energies.

02.1 Explain why beryllium is the element with the highest first ionisation energy in

Group 2.

[2 marks]

02.2 Give an equation, including state symbols, to show the process that occurs when

the second ionisation energy of beryllium is measured.

[1 mark]

02.3 Predict which element in Period 3 has the lowest second ionisation energy.

[1 mark]

02.4 Figure 1 shows a plot of log10(ionisation energy) for a Period 3 element.

The ionisation energies are measured in kJ mol–1

Figure 1

*05Deduce the identity of this element.*

Explain your answer.

[3 marks]

Element

Explanation

02.5 Use Figure 1 to determine the third ionisation energy, in kJ mol–1, of this element.

[1 mark]

kJ mol–1

Mark scheme

Show the mark scheme

Question Answers Additional comments/Guidelines Mark

M1 smallest atom/atomic radius

Allow comparative and superlative only for M1

and M2

smallest number of shells/energy levels

OR Allow correct trends up or down Group 2

outer electron is closest to the nucleus

02.1 OR

(2 x AO1)

least shielding

M2 strongest attraction between outer/valence electron(s) and

nucleus

Be+(g) → Be2+(g) + e–

02.2 OR

(1 x AO1)

Be+(g) + e– → Be2+(g) + 2 e–

Mg/Magnesium 1

02.3

(1 x AO3)

– A-LEVEL CHEMISTRY – –

M1 P Mark M1, M2 and M3 independently

M2 large increase/jump from 5th to 6th ionisation energy/after 5th 3

ionisation energy

02.4 (2 x AO1,

1 x AO2)

M3 the 6th electron is removed from the 2 (principal) energy

level/from a lower energy level/from a lower shell/from 2p/from an

energy level that is closer to the nucleus

allow answer in range 2500–3200 1

02.5

(1 x AO3)

How to answer it

Mastering Ionisation Energies

What this question tests

This question assesses your understanding of periodicity and atomic structure . Specifically, it tests your ability to explain trends in first ionisation energies down Group 2, write precise equations for successive ionisation energies with state symbols, predict second ionisation energy anomalies in Period 3, identify elements from successive ionisation energy graphs, and perform logarithmic scale conversions.

Question 02.1

Group 2 First Ionisation Energy Trend

Explain why beryllium is the element with the highest first ionisation energy in Group 2. [2 marks]

✅ Model Answer

Beryllium has the smallest atomic radius (or has the fewest shells / least shielding). [1 mark]
Therefore, it has the strongest attraction between the nucleus and the outer/valence electrons. [1 mark]

💡 Key Knowledge

As you go down Group 2:
• Extra electron shells are added, increasing the atomic radius.
• Inner shells shield the outer electrons from the nuclear charge (shielding increases).
• Even though nuclear charge increases, the increased distance and shielding dominate, making outer electrons easier to remove.

🧠 Exam Technique

Always structure trend explanations in two distinct steps:
1. Physical structure: Mention size/radius, shielding, or distance.
2. Electrostatic force: Connect this structure to the strength of attraction between the positive nucleus and the negative outer electron.
Tip: Use comparative/superlative terms (e.g., "smallest", "strongest") because you are comparing beryllium to the rest of the group.

❌ Common Errors

• Stating that beryllium has "more protons" as the reason. While true, nuclear charge is offset by shielding down a group.
• Forgetting to specify that the attraction is between the nucleus and the outer electron. Simply saying "stronger attraction" is too vague to score.

Question 02.2

Writing Successive Ionisation Equations

Give an equation, including state symbols, to show the process that occurs when the second ionisation energy of beryllium is measured. [1 mark]

✅ Model Answer

Be⁺(g) → Be²⁺(g) + e⁻

Alternative accepted: Be⁺(g) + e⁻ → Be²⁺(g) + 2e⁻

[1 mark]

💡 Key Knowledge

The nth ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous (n-1)⁺ ions to form one mole of gaseous n⁺ ions.
• For the second ionisation energy, you must start with a 1+ gas ion and end with a 2+ gas ion.

❌ Common Errors

• Missing state symbols: Ionisation energy is strictly defined for gaseous species. Forgetting (g) is an automatic zero. • Wrong starting species: Writing Be(g) → Be²⁺(g) + 2e⁻ . This represents the sum of the first and second ionisation energies, not the second ionisation energy itself.

Question 02.3

Period 3 Second Ionisation Energy

Predict which element in Period 3 has the lowest second ionisation energy. [1 mark]

✅ Model Answer

Mg (or Magnesium)

[1 mark]

💡 Key Knowledge

Why is it Magnesium?
• Sodium (Na): 1st IE removes the 3s¹ electron. The 2nd IE must remove an electron from the stable, lower-energy 2p⁶ shell, which requires a massive amount of energy.
• Magnesium (Mg): 1st IE removes a 3s electron. The 2nd IE removes the second 3s electron. This electron is in the outer shell, shielded by the inner shells, making it relatively easy to remove.
• As you move from Mg to Ar, the 2nd IE increases due to increasing nuclear charge. Thus, Mg is the lowest.

Question 02.4

Deducing Identity from Successive Ionisation Energies

Deduce the identity of the Period 3 element shown in Figure 1. Explain your answer. [3 marks]

✅ Model Answer

Element: P (or Phosphorus) [1 mark]
Explanation 1: There is a large increase (jump) between the 5th and 6th ionisation energies. [1 mark]
Explanation 2: This indicates that the 6th electron is being removed from a lower principal energy level (inner shell / 2p orbital) which is closer to the nucleus. [1 mark]

🧠 Exam Technique

To secure all 3 marks, follow this logical chain:
1. Identify: State the element symbol or name clearly.
2. Locate the Jump: Explicitly state between which two successive ionisation numbers the big jump occurs (e.g., "between 5th and 6th").
3. Explain the Jump: Explain that the 6th electron is being taken from an inner shell (or lower energy level) which experiences much stronger electrostatic attraction to the nucleus.

Question 02.5

Interpreting Logarithmic Scales

Use Figure 1 to determine the third ionisation energy, in kJ mol⁻¹, of this element. [1 mark]

✅ Model Answer

Any value in the range 2500 to 3200 kJ mol⁻¹ is accepted.

[1 mark]

📐 Calculation Steps

Locate the data point: Find 3 on the x-axis (Ionisation number).
Read the y-axis value: Follow the grid line up to the plotted point. The y-axis value (log₁₀ of IE) is approximately 3.45 (acceptable range: 3.40 to 3.50).
Perform the inverse log: To find the actual value, calculate 10 raised to the power of your reading:
IE = 10³.⁴⁵ = 2818 kJ mol⁻¹
• If you read 3.40: 10³.⁴⁰ = 2512 kJ mol⁻¹
• If you read 3.50: 10³.⁵⁰ = 3162 kJ mol⁻¹

❌ Common Errors

• Forgetting the inverse log: Writing down the raw y-axis value (e.g., 3.45) as the final answer. Remember, the y-axis is plotted as log₁₀(IE) , not the actual IE!
• Incorrect math operation: Multiplying by 10 or doing 3.45 x 1000. You must use the base-10 power function ( 10^x ) on your calculator.

Topics

Physical Chemistry · Inorganic Chemistry · 3.1.1 Atomic Structure · 3.2.1 Periodicity · 3.2.2 Group 2, The Alkaline Earth Metals

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.