Edexcel A-Level Chemistry Paper 1, November 2021: Question 4

15 marks · Medium difficulty · Short Open Response

Assess and analyze successive ionisation energies, definitions, trends down Group 1, and graphical data for magnesium.

Practise this question

Question

Question text

4 Ionisation energies provide information about the number of electrons and the

arrangement of the electrons in an atom of an element.

(a) A student’s definition of first ionisation energy is shown.

First ionisation energy is the energy released when one mole of gaseous atoms

loses one mole of electrons to form one mole of gaseous 1+ ions.

There is one incorrect word in the student’s definition.

Identify the word, giving the reason why this word is incorrect.

(2)

(b) Write an equation for the second ionisation energy of oxygen.

Include state symbols.

(2)

Explain why the first ionisation energy of sodium is greater than that of potassium.

(3)

(d) The successive ionisation energies for magnesium are given in the table.

Electron6

number 1 2 3 *P65463A0628*456789 10 11 12

removed

Ionisation

energy 738 1451 7733 10541 13629 17995 21704 25657 31644 35463 169996 189371

/ kJ mol–1

Log

(ionisation 2.87 3.16 3.89 4.02 4.13 4.34 4.41 4.50 4.55 5.23

energy)

(i) Complete the table.

(1)

(ii) Give a reason why the logarithm of the ionisation energy, rather than just the

ionisation energy, is used to plot a graph.

(1)

… *P65463A0728*

(iii) Give a reason why the successive ionisation energies increase.

(1)

(iv) Plot the graph of log(ionisation energy) against electron number removed.

Join the individual points using straight lines.

(3)

*P65463A0828*

(v) Identify on the graph, using a circle, the points that represent the removal of

the electrons in the outermost energy level of magnesium.

(1)

(e) Estimate a value for the first ionisation energy of oxygen given the data in the table.

(1)

Element First ionisation energy / kJ mol–1

carbon 1086

nitrogen*P65463A0928*1402

oxygen …

(Total for Question 4 = 15 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

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

• (energy) released (1)

• energy is required to overcome the (electrostatic) attraction from Allow energy is required to remove an

the nucleus for the electron (1) electron

Allow (the removal of an electron) is

endothermic

Question

Answer Additional Guidance Mark

Number

4(b) An answer that includes Example of equation (2)

• species in suitable equation (1) O+(g) → O2+(g) + e(-)

ALLOW

O+(g) − e(-) → O2+(g)

• state symbols (1)

Ignore state symbols on electron

Do not allow multiples for M1

M2 dependent on M1 or near miss

Question

Answer Additional Guidance Mark

Number

4(c) An explanation that makes reference to the following points Accept reverse arguments throughout (3)

• the outer electron in a sodium atom is closer to the nucleus (than Allow sodium atoms are smaller (than

that in potassium) (1) potassium)

Allow sodium has electron in 3s whereas

potassium has electron in 4s

Allow diagram to illustrate

Do not award reference to ionic radius

• (and) less shielding from inner electron shells (1)

• these outweigh the greater nuclear charge / number of protons in Do not award if reference given to both

potassium (1) have a +1 charge/ same nuclear charge

Question

Answer Additional Guidance Mark

Number

4(d)(i) (1)

• completed table 4.26 and 5.28

Question

Answer Additional Guidance Mark

Number

4(d)(ii) An answer that includes (1)

Allow:

• the range of numbers / 738 to 189 371 is too large (to fit on a graph / a (very) long y axis would be needed

axis) (some of) the numbers are too large

logarithms make it easier to plot the numbers the difference between the ionisation

energies is too large

so the numbers will fit on the graph

logs give smaller (range of) numbers

Ignore simpler to read

Do not award reference to averages

Question

Answer Additional Guidance Mark

Number

4(d)(iii) An answer that includes one of the following points (1)

• the same number of protons is attracting a decreasing number of

electrons

electron is removed from an increasingly positively charged ion Do not award each electron is removed

or from shells closer to the nucleus

electron removed is closer to the nucleus

the electron removed is experiencing less electron-electron

repulsion

Question

Answer Additional Guidance Mark

Number

4(d)(iv) An answer that includes a suitable graph (3)

• labelled axes (1) Do not award if any units given on y axis

• suitable scale (1) Plotted points must cover at least ½ the graph

paper on each axis

• plotting of points and lines joining points (1) Allow ±½ square

Do not award line going from point 1 to the

origin

Ignore plotting of electron 6 and 12

Exemplar graph

Question

Answer Additional Guidance Mark

Number

4(d)(v) An answer that includes Exemplar circle on graph (1)

• circle around the first two points/circles around individual points

Question

Answer Additional Guidance Mark

Number

4(e) An answer that includes (1)

• (estimated value) between 1100 – 1380 (kJ mol—1)

(Total Question 4 = 15 marks)

How to answer it

Ionisation Energies and Electronic Structure

📌 What this question tests

This question evaluates your core understanding of ionisation energies. You will be tested on definitions, writing ionic equations with correct state symbols, explaining periodic trends down a group, interpreting successive ionisation energy data using logarithms, graphical skills, and predicting trends across periods (specifically looking at the general increase in first ionisation energy across Period 2).

Part (a): Evaluating a Definition

✅ Correct Answer

Incorrect word: released

Reason: Energy is required (absorbed) to overcome the electrostatic attraction between the positive nucleus and the negative electron being removed (making ionisation endothermic).

❌ Common Errors

Students often incorrectly target words like "atoms" or "gaseous". Make sure you remember that removing an electron always requires energy input because you are working against attractive forces.

🎯 2 Marks: 1 mark for identifying "released", 1 mark for stating energy is required to overcome nuclear attraction.

Part (b): Equation for Second Ionisation Energy

✅ Correct Answer

O⁺(g) → O²⁺(g) + e⁻

(Also accepted: O⁺(g) + e⁻ → O²⁺(g) written backwards, or omitting state symbols on the electron).

💡 Key Knowledge

The second ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions. State symbols ( g ) are mandatory for all species except electrons.

🎯 2 Marks: 1 mark for correct species and charges, 1 mark for complete correct state symbols. Note: M2 is dependent on getting M1 right.

Part (c): Explaining Trends Down Group 1

✅ Correct Answer

The outer electron in sodium is in a principal energy level closer to the nucleus than in potassium ( 3s vs 4s ), resulting in less shielding from inner electron shells. These factors outweigh the greater nuclear charge of potassium.

🧠 Exam Technique

When comparing elements down a group, always structure your answer around three pillars: atomic radius/distance, shielding, and nuclear attraction. Explicitly state which factor wins!

❌ Common Errors

Do not just state that potassium has more protons and stop there; you must explain how the increased distance and shielding override the extra nuclear charge.

🎯 3 Marks: 1 mark for distance/smaller radius in Na, 1 mark for less shielding in Na, 1 mark for stating these factors outweigh potassium's extra protons.

Part (d): Successive Ionisation Energies of Magnesium

✅ Correct Answers & Calculations

(i) Missing Log Values: For electron 6: 4.26 . For electron 12: 5.28 .
(ii) Why use Logs?: The range of numbers (738 to 189 371) is too large to fit conveniently on a standard graph axis.
(iii) Why successive I.E. increases: Each successive electron is removed from an increasingly positively charged ion (or the same number of protons is attracting fewer remaining electrons, reducing electron-electron repulsion).

📐 Log Calculation Walkthrough

To find the missing log values, apply base-10 logarithm ( log₁₀ ) to the given ionisation energy:

For value 17 995: log₁₀(17995) = 4.255 → round to 4.26
For value 189 371: log₁₀(189371) = 5.277 → round to 5.28

📊 Sub-parts (iv) & (v): Graph Plotting & Interpretation

(iv) Plotting: Ensure axes are fully labelled (Electron number removed vs Log ionisation energy). Use a suitable scale covering at least half of the grid. Join points with straight lines, but do not extrapolate a line back to the origin.

(v) Outermost Electrons: Draw a circle around the first two points (electrons 1 and 2). This highlights the two valence electrons in magnesium's outer 3s sub-shell before a massive jump occurs at electron 3.

🎯 Total 6 Marks across parts (i) to (v): 1 for table completion, 1 for log reason, 1 for increase reason, 3 for graph execution, 1 for outer electron identification.

Part (e): Estimating First Ionisation Energy

✅ Correct Answer

Any value between 1100 and 1380 kJ mol⁻¹ .

💡 Key Knowledge

First ionisation energy generally increases across a period (from carbon to nitrogen to oxygen). Although oxygen exhibits a slight drop due to spin-pairing repulsion in its 2p sub-shell, its value remains higher than carbon's (1086) and close to or just below nitrogen's (1402).

🎯 1 Mark: For providing a realistic estimate within the acceptable tolerance range.

Topics

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

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.