Edexcel A-Level Chemistry Paper 3, June 2025: Question 1

7 marks · Medium difficulty · Open Response

Define first ionisation energy, deduce the group to which elements X, Y, and Z belong using successive ionisation energy data, and explain which element has the highest atomic number.

Practise this question

Question

Question text

1 This question is about ionisation energies.

The table shows the first four ionisation energies of three elements in the same group

of the Periodic Table.

The letters used are not the symbols of the elements.

First ionisation Second ionisation Third ionisation Fourth ionisation

Element energy energy energy energy

/ kJ mol−1 / kJ mol−1 / kJ mol−1 / kJ mol−1

X 403 2632 3900 5080

Y 419 3051 4412 5877

Z 496 4563 6913 9544

(a) State what is meant by the term ‘first ionisation energy’.

(2)

(b) Explain to which group the elements X, Y and Z belong.

(2)

(3)

… *P77834A0236*

(Total for Question 1 = 7 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

1(a) An answer that makes reference to the following points: (2)

• energy (required) to remove / lose 1 (1) Allow e(−) for electron

electron / first electron / last electron from

an / each atom (of an element)

• in 1 mol (e) of gaseous atoms (to form 1 mol (1)

of positive ions)

1 mol(e) must be referenced in either M1 or M2

to score both marks

e.g. ‘energy to remove 1 mol of electrons from

gaseous atoms’ scores 2 marks

Correct example symbol or generic equation, for

first IE, including state symbols scores 1 mark

e.g. X(g) → X+(g) + e(–) / X(g) – e(–) → X+(g)

Ignore references to standard conditions /

standard states

Question

Answer Additional Guidance Mark

Number

1(b) An explanation that makes reference to the following points: Allow there is a very large increase in 2nd IE (2)

• as there is a large difference / large increase / (1)

(large) jump / in ionisation energy between first and

second ionisation energy / after first ionisation energy /

before second ionisation energy Allow alkali metal

• Group 1 (1) M2 dependent on M1

Ignore reference to the subshell / orbital of the

(outer) electron even if incorrect

Question

Answer Additional Guidance Mark

Number

1(c) An explanation that makes reference to the following points: Penalise ionic radius once only (3)

• element X as it has the lowest (first) ionisation energy

(1) Allow X as the (first) ionisation energy

decreases down the group

Do not award reference to other ionisation

• as electron removed is furthest from nucleus energies (unless all are mentioned as being

lower)

(1)

Allow ‘outer electron’ for ‘electron removed’

Allow atomic radius is greater

Ignore more shells

• so there is greater shielding (even though there is an Do not award ionic radius

increase in the number of protons)

(1)

Allow greater repulsion from inner electron

shells

Allow 1 mark for there is less attraction

between the nucleus and the outer electron if

neither M2 or M3 is awarded.

Allow 1 mark for ‘Z, because the nuclear

charge / number of protons is the greatest’

Ignore effective nuclear charge

(Total for Question 1 = 7 marks)

How to answer it

Successive Ionisation Energies & Group Trends

Edexcel A-Level Chemistry • Core Principles: Atomic Structure & Periodicity

📌 What this question tests

This question evaluates your foundational understanding of atomic structure and periodic trends:

Precision in definitions: Recalling the strict, unambiguous textbook definition of first ionisation energy.
Analysing successive IE data: Identifying large energy jumps to deduce valence electron configuration and group number.
Down-group periodic trends: Explaining variations in nuclear attraction using atomic radius, shielding, and nuclear charge.

Provided Exam Data

Element	1st IE / kJ mol⁻¹	2nd IE / kJ mol⁻¹	3rd IE / kJ mol⁻¹	4th IE / kJ mol⁻¹
X	403	2632	3900	5080
Y	419	3051	4412	5877
Z	496	4563	6913	9544

Part (a) • 2 Marks

Definition of First Ionisation Energy

State what is meant by the term 'first ionisation energy'.

✅ Model Answer

The energy required to remove one electron from each atom in one mole of gaseous atoms (to form one mole of gaseous 1+ ions).

Mark Scheme Breakdown:
• [1 mark]: Energy needed to remove 1 electron from each atom.
• [1 mark]: In 1 mole of gaseous atoms.
Note: Mentioning "1 mole" in either statement is compulsory to unlock both marks.

🧠 Exam Technique & Equation Fallback

Always include key terms: "Energy required", "1 mole", "gaseous", and "atoms".
Symbolic representation: A fully correct state-annotated equation scores 1 mark if your text definition is incomplete:
X(g) → X⁺(g) + e⁻
Do not mention standard conditions; ionisation energy is defined strictly in the gas phase.

❌ Common Errors

Missing the state: Writing "from an atom" instead of "from a gaseous atom".
Confusing quantities: Writing "molecules" instead of "atoms".
Omitting the mole: Saying "energy to remove one electron from a gaseous atom" only secures 1 of the 2 marks.

Part (b) • 2 Marks

Deducing the Group Number

Explain to which group the elements X, Y and Z belong.

✅ Model Answer

There is a large jump (large increase) between the first and second ionisation energies for all three elements.
Therefore, elements X, Y and Z belong to Group 1 (alkali metals).

Mark Scheme Breakdown:
• [1 mark]: Identifying the large increase between the 1st and 2nd IE.
• [1 mark]: Stating Group 1 (dependent on identifying the jump).

💡 Key Knowledge

A massive jump in ionisation energy occurs when an electron is removed from a new principal quantum shell closer to the nucleus with significantly less shielding. Since only 1 electron is easily removed before the jump, there is 1 valence electron, placing the elements in Group 1.

🧠 Exam Technique

Always state between which successive values the jump occurs. Never just say "there is a big jump in the numbers". You must specify: "between the 1st and 2nd ionisation energy".

Part (c) • 3 Marks

Determining Relative Atomic Number Down a Group

Explain which of the elements X, Y and Z has the highest atomic number.

✅ Model Answer

Element X has the highest atomic number because it has the lowest first ionisation energy (403 kJ mol⁻¹).
Its outer electron is furthest from the nucleus (largest atomic radius / most electron shells).
There is greater shielding from inner shells, which outweighs the increase in nuclear charge, resulting in weaker nuclear attraction for the outer electron.

Mark Scheme Breakdown:
• [1 mark]: Identifies Element X due to having the lowest 1st IE (IE decreases down the group).
• [1 mark]: Outer electron is furthest from the nucleus (greater atomic radius / more shells).
• [1 mark]: Greater electron shielding (outweighs increased nuclear charge).

💡 The Three Competing Factors

Whenever explaining group IE trends, evaluate:

Atomic Radius: Increases down the group as extra quantum shells are added.
Shielding: Increases significantly due to additional full inner electron shells.
Nuclear Charge: Increases (more protons), but this is completely overridden by the increased distance and shielding.

❌ Common Errors & Examiner Traps

Saying "ionic radius": Automatic zero for that point! You are ionising neutral gaseous atoms, so you must refer to atomic radius or distance from the outer electron to the nucleus.
Choosing Element Z: Confusing the trend and assuming higher IE means higher atomic number. In a group, atomic number increases downwards while IE decreases.
Vague "nuclear charge" claims: Simply stating "nuclear charge increases" without explaining that shielding and radius dominate will lose marks.

Topics

Physical Chemistry · Inorganic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 4: Inorganic Chemistry and the Periodic Table

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