AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2020: Question 7

11 marks · Standard Demand difficulty · Extended Answer

Explain electron transfer in the formation of caesium oxide, explain why caesium is more reactive than sodium, and explain why Mendeleev placed tellurium before iodine in the periodic table.

Practise this question

Question

The question page shows Question 7 about elements, split into three written-response parts with lined answer spaces. Part 07.1 states that caesium is in Group 1 of the periodic table and asks students to explain, in terms of electrons, what happens to caesium atoms and oxygen atoms when caesium reacts with oxygen to produce caesium oxide for 4 marks. Part 07.2 asks, again in terms of electrons, why caesium is more reactive than sodium for 4 marks. Part 07.3 includes a small figure labelled Figure 6 showing two columns from Mendeleev’s periodic table: left column O 16, S 32, Se 79, Te 128 and right column F 19, Cl 35.5, Br 80, I 127; it asks why early periodic tables placed iodine before tellurium but Mendeleev placed tellurium before iodine for 3 marks.
Question text

07 This question is about elements.

Caesium is in Group 1 of the periodic table.

07.1 Explain what happens to caesium atoms and to oxygen atoms when caesium reacts

with oxygen to produce caesium oxide.

You should answer in terms of electrons.

[4 marks]

07.2 Explain why caesium is more reactive than sodium.

You should answer in terms of electrons.

[4 marks]

07.3 Figure 6 shows part of Mendeleev’s periodic table.

Figure 6

Explain why the early periodic tables placed iodine (I) before tellurium (Te), but then

Mendeleev placed tellurium before iodine.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme is a table with columns for question number, answers, extra information, mark, and AO/specification reference, giving a total of 11 marks. For 07.1 it credits that a caesium atom loses one electron, an oxygen atom gains two electrons, two caesium atoms react with one oxygen atom, and formation of Cs+ and O2- or ions with full outer shells. For 07.2 it credits that caesium has more shells or energy levels, so the outer electron is further from the nucleus or more shielded, there is weaker attraction, and the outer electron is more easily lost. For 07.3 it credits that early periodic tables were arranged by atomic weight, iodine has a lower atomic weight than tellurium, and Mendeleev placed elements with similar properties together so tellurium was placed with oxygen, sulfur and selenium while iodine was placed with fluorine, chlorine and bromine.

AO /

Question Answers Extra information Mark

Spec. Ref.

07.1 max 3 marks if reference to

incorrect particles / bonding /

structure

caesium atom loses one 1 AO1

electron 5.1.2.5

5.2.1.2

(and) oxygen atom gains two 1

electrons

(so) two caesium atoms react allow (to produce) Cs2O 1

with one oxygen atom

any one from: 1

• (to form) Cs+ and O2-

• (to form) caesium ion(s) and

oxide ion(s)

• (to form) ions with full outer

shells / levels

07.2 allow converse for sodium

(caesium has) more energy 1 AO2

levels

or

(caesium has) more shells

(so the) outer electron / shell is 1 AO1

further from nucleus

or

outer electron / shell is more

shielded

(so) weaker attraction between 1 AO1

nucleus and outer electron /

shell

(so) outer electron is more allow (so) less energy needed to 1 AO1

easily lost remove outer electron

5.1.2.3

5.1.2.5

07.3 early periodic tables were ignore atomic mass 1 AO1

arranged with elements in order

of their atomic weights

iodine has a lower atomic weight allow converse for tellurium 1 AO1

than tellurium

(so) Mendeleev placed iodine allow F / Cl / Br for elements 1 AO2

with elements with same /

similar properties 5.1.2.2

or

(so) Mendeleev placed tellurium allow O / S / Se for elements

with elements with same /

similar properties

Total 11

How to answer it

Standard Demand

Group 1 reactivity and Mendeleev’s periodic table

What this question tests
You need to explain reactions using electrons, show how ions form, and use periodic table patterns. It also tests why reactivity increases down Group 1 and why atomic mass was not always the deciding factor in early periodic tables.

Question 07.1: Caesium reacting with oxygen

Explain what happens to caesium atoms and oxygen atoms when caesium reacts with oxygen to produce caesium oxide.

✅ Correct answer

  • Caesium atoms lose one electron.
  • Oxygen atoms gain two electrons.
  • Therefore two caesium atoms react with one oxygen atom.
  • This forms Cs⁺ and O²⁻ ions with full outer shells.
Mark idea: 1 mark for electron loss, 1 for oxygen gaining electrons, 1 for the 2:1 ratio / Cs₂O, 1 for ions or full outer shells.

💡 Key knowledge

  • Group 1 metals each have 1 electron in the outer shell.
  • Oxygen needs 2 electrons to fill its outer shell.
  • Ion formation happens so atoms get full outer shells.
  • Caesium oxide is made of Cs⁺ and O²⁻ ions.

🧠 Exam technique

  • Use the word electrons clearly — the question tells you to answer in terms of electrons.
  • Link the electron transfer to the formula: oxygen needs two electrons, so two caesium atoms are needed.
  • Short, clear chemistry terms score best: loses, gains, ions, full outer shell.

❌ Common errors

  • Talking about bonding instead of electrons.
  • Saying oxygen loses electrons.
  • Forgetting the 2:1 ratio between caesium and oxygen.
  • Writing vague phrases like “they join together” without electron detail.

📐 Step-by-step reaction idea

  1. Each caesium atom loses 1 electron → Cs⁺.
  2. One oxygen atom needs 2 electrons → O²⁻.
  3. So 2 caesium atoms are needed to supply 2 electrons.
  4. Final formula: Cs₂O.

Question 07.2: Why caesium is more reactive than sodium

Explain why caesium is more reactive than sodium. Answer in terms of electrons.

✅ Correct answer

  • Caesium has more energy levels / more shells than sodium.
  • Its outer electron is further from the nucleus and/or more shielded.
  • There is a weaker attraction between the nucleus and the outer electron.
  • So the outer electron is lost more easily and less energy is needed to remove it.
Top answers linked all four ideas: more shells → more shielding/further away → weaker attraction → easier electron loss.

💡 Key knowledge

  • All Group 1 metals react by losing 1 outer electron.
  • Reactivity increases down Group 1 because the outer electron becomes easier to remove.
  • Caesium is lower in the group than sodium, so it has more shells.

🧠 Exam technique

  • Use a chain of reasoning, not just one fact.
  • Good sequence: more shells → more shielding → weaker attraction → easier to lose electron.
  • If you only say “caesium is more reactive because it is lower down the group,” that is not enough for full marks.

❌ Common errors

  • Saying the outer electron is closer to the nucleus.
  • Talking about mass rather than electron structure.
  • Writing that caesium “wants” to react more — this is too vague.
  • Forgetting to mention the electron is lost more easily.

Question 07.3: Iodine and tellurium in Mendeleev’s table

Explain why the early periodic tables placed iodine before tellurium, but Mendeleev placed tellurium before iodine.

✅ Correct answer

  • Early periodic tables were arranged in order of atomic weight.
  • Iodine has a lower atomic weight than tellurium, so it came first.
  • Mendeleev changed the order to keep elements with similar properties together.
  • So he placed tellurium before iodine.
The key mark was for recognising that Mendeleev prioritised properties over strict atomic weight order.

💡 Key knowledge

  • Modern periodic tables are based on atomic number, but this question is about the old order used by early tables.
  • Mendeleev grouped elements by similar chemical properties.
  • This explains why he sometimes put elements in an order that did not match atomic weight exactly.

🧠 Exam technique

  • Make sure you answer both parts of the question: why early tables put iodine first, and why Mendeleev reversed it.
  • Use the phrase similar properties — this is the key idea examiners look for.
  • Examples from the same groups can help: F / Cl / Br or O / S / Se, but they are not required here.

❌ Common errors

  • Saying Mendeleev used atomic number — he did not.
  • Only mentioning atomic weight and forgetting similar properties.
  • Mixing up which element has the lower atomic weight.

📐 What to remember from the table

  • Iodine = atomic weight 127
  • Tellurium = atomic weight 128
  • Early tables: lightest first
  • Mendeleev: similar properties first, even if the masses were slightly out of order

How the marks were awarded

07.1: 4 marks

  • Caesium loses one electron.
  • Oxygen gains two electrons.
  • Two caesium atoms react with one oxygen atom / Cs₂O.
  • Cs⁺ and O²⁻ or full outer shells.

07.2: 4 marks

  • More shells / energy levels.
  • Outer electron further from nucleus / more shielding.
  • Weaker attraction.
  • Outer electron lost more easily / less energy needed.

07.3: 3 marks

  • Early tables ordered by atomic weight.
  • Iodine had lower atomic weight than tellurium.
  • Mendeleev grouped by similar properties, so Te before I.

Quick final revision points

✅ Must-know facts

  • Group 1 metals lose 1 electron.
  • Oxygen gains 2 electrons.
  • Down Group 1, reactivity increases.
  • Mendeleev used similar properties, not just atomic weight.

❌ Final trap check

  • Do not confuse mass with reactivity.
  • Do not say oxygen gains one electron.
  • Do not forget the 2:1 ratio for caesium oxide.

Topics

Chemistry · C1: Atomic Structure and the Periodic Table · C2: Bonding, Structure and the Properties of Matter

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.