Edexcel A-Level Chemistry AS Paper 1, June 2018: Question 8
9 marks · Hard difficulty · Open Response
Analyze ionic bonding, including equations for formation, dot-and-cross diagrams, trends in ionic radius across isoelectronic ions, and the effect of cation charge and radius on ionic bond strength.
Practise this questionQuestion
Question text
8 This question is about ionic bonding.
(a) The elements sodium and fluorine react together to form an ionic compound.
(i) Select the correct equation for this reaction.
(1)
A Na(s) + F(g) → NaF(s)
B 2Na(s) + F2(g) → 2NaF(s)
C Na(s) + F2(g) → NaF2(s)
D 2Na(s) + F(g) → Na2F(s)
(ii) Draw dot-and-cross diagrams of the ions in sodium fluoride, showing all the electrons.
Use your diagram to explain why the ions are described as isoelectronic.
(3)
(iii) Which diagram shows the trend in ionic radius for the isoelectronic ions N3− to Al3+?
(1)
A B
Ionic Ionic
radius radius
N3− O2− F− *P51459A02024*(Ne) Na+Mg2+Al3+N3−O2− F− (Ne) Na+ Mg2+ Al3+
C D
Ionic Ionic
radius radius
N3− O2− F− (Ne) Na+ Mg2+ Al3+ N3− O2− F− (Ne) Na+ Mg2+ Al3+
(iv) Explain your answer to (a)(iii) in terms of the structure of the ions.
(2)
(b) The strength of ionic bonding in different compounds can be compared by using*P51459A02124*
the amount of energy required to separate the ions. Some values for this energy
are given in the table.
Amount of energy required to separate the ions
Compound −1
/ kJmol
LiF 1031
KF 817
CaF2 2957
Using the data provided, explain how changes in the cation affect the bond
strength in an ionic compound.
(2)
(Total for Question 8 = 9 marks)
Mark scheme
Show the mark scheme
Question
Answer Mark
Number
8(a)(i) The only correct answer is B (1)
A is not correct because fluorine is diatomic
C is not correct because sodium is 1+ ion
D is not correct because fluorine is diatomic
Question
Acceptable Answer Additional Guidance Mark
Number
8(a)(ii) A diagram which shows the first two points: (3)
Example of diagram
electronic configuration for Na is 2.8 and +1 charge (1)
electronic configuration for F is 2.8 and -1 charge (1)
Allow one mark if both ions have eight
electrons in their outer shell if M1 and
M2 not scored
OR
Both with correct charge if M1 and M2
not scored.
Do not award either mark for a
covalent bond
Ignore balancing numbers
isoelectronic ions have the same electronic configuration Allow same number of electrons
(1)
Question
Answer Mark
Number
8(a)(iii) The only correct answer is A (1)
B is not correct because diagram has cations larger than anions
C is not correct because diagram has cations larger than anions
D is not correct because trends in wrong direction
Question
Acceptable Answer Additional Guidance Mark
Number
8(a)(iv) (2)
increase in number of protons (in the nucleus) (1) Allow increasing nuclear charge
increases the attraction for the electrons (bringing them For explanations of graph B allow max
closer to the nucleus) (1) (1) for a correct explanation for any
downward trend for three ions
Allow max (1) for an explanation of
the smallest or largest ion without an
explanation of the trend
e.g. Al3+ has the most protons so
electrons most attracted to nucleus so
smallest scores (1)
Discussion of atomic radius max (1)
Question
Acceptable Answer Additional Guidance Mark
Number
8(b) An explanation that makes reference to the following Allow “stronger bonding” for stronger (2)
points: attraction between ions
the higher the charge on the cation the stronger the Both charges should be stated
attraction between ions and mention of a 2+ cation in Allow calcium ions have twice the
CaF2 compared to a 1+ cation in LiF / KF (1) charge of potassium / lithium ions.
the smaller the radius of the cation the stronger the
attraction between ions and mention of Li+ being smaller Do not award ‘lithium has a smaller
than K+ (1) radius than potassium’ unless it is
clear ions are being considered, for
example the use of Li+ and K+ in the
answer.
If no other marks awarded, allow a
discussion of charge density without
reference to charge or radius of one
pair of ions for (1)
If no other mark awarded, allow a
correct statement about the effect of
charge and ionic radius without
justification from table of data for (1)
(Total for Question 8 = 9 marks)
TOTAL FOR PAPER = 80 MARKS
How to answer it
Ionic Bonding, Isoelectronic Ions, and Lattice Energy Trends
This question assesses your understanding of ionic bonding formation, constructing dot-and-cross diagrams for ions, explaining the concept of isoelectronic species, interpreting graphical trends in ionic radius across isoelectronic series (N³⁻ to Al³⁺), and evaluating how cationic charge and radius affect ionic bond strength/lattice energy.
Equation for Sodium and Fluorine Reaction
✅ Correct Answer
B: 2Na(s) + F₂(g) → 2NaF(s)
❌ Common Errors
- Option A & C are incorrect because fluorine is a diatomic gas ( F₂ ), not monatomic.
- Option C and D incorrectly balance formulas or ionic charges (sodium forms a 1+ ion, so formula is NaF , not NaF₂ or Na₂F ).
Dot-and-Cross Diagrams and Isoelectronic Ions
💡 Key Knowledge
To score full marks, your diagram must show:
- Sodium ion ( Na⁺ ) with electronic configuration 2.8 and a + charge outside brackets.
- Fluoride ion ( F⁻ ) with electronic configuration 2.8 (8 electrons in outer shell) and a - charge outside brackets.
- An explicit definition or statement explaining that isoelectronic ions have the same electronic configuration (both have 10 electrons / 2.8 structure).
❌ Common Errors
- Drawing a covalent bond instead of separate ions with brackets and charges.
- Omitting square brackets or charges.
- Failing to explicitly link the diagram to the definition of 'isoelectronic' (having identical electron arrangements).
Trend in Ionic Radius for Isoelectronic Ions (N³⁻ to Al³⁺)
✅ Correct Answer
Graph A
Graph A correctly shows a continuous downward trend in ionic radius from N³⁻ down to Al³⁺, split across the noble gas neon gap.
🧠 Exam Technique
Remember that across an isoelectronic series, nuclear charge increases while the number of shielding electrons remains constant. Therefore, ionic radius must steadily decrease as you move from left to right across the period.
Explaining the Trend in Ionic Radius
💡 Key Knowledge
To secure both marks, your explanation must include:
- Point 1: An increase in the number of protons in the nucleus (or increasing nuclear charge).
- Point 2: This results in a stronger electrostatic attraction for the electrons, pulling them closer to the nucleus and reducing the ionic radius.
❌ Common Errors
- Discussing 'atomic radius' instead of 'ionic radius'.
- Giving a partial explanation mentioning only the smallest or largest ion without describing the overarching trend of nuclear charge.
Effect of Cation Charge and Radius on Bond Strength
📐 Step-by-Step Analysis of Data
Compare the given energy values: LiF (1031 kJ mol⁻¹) , KF (817 kJ mol⁻¹) , and CaF₂ (2957 kJ mol⁻¹) .
- Step 1 (Charge effect): Compare KF (K⁺ is 1+) and CaF₂ (Ca²⁺ is 2+). The higher the charge on the cation ( Ca²⁺ has twice the charge of K⁺ ), the stronger the electrostatic attraction between ions, requiring more energy to separate them.
- Step 2 (Radius effect): Compare LiF and KF (both have 1+ cations). The smaller the radius of the cation ( Li⁺ is smaller than K⁺ ), the closer the ions can get, leading to stronger attraction and higher bond strength.
❌ Common Errors
- Speaking generally about "ions" without explicitly naming the specific ions from the data table (e.g., you must reference Li⁺ , K⁺ , or Ca²⁺ ).
- Confusing cationic radius trends (remember: down a group, ionic radius increases, so K⁺ is larger than Li⁺).
Topics
Physical Chemistry · Inorganic Chemistry · Topic 2: Bonding and Structure · Topic 4: Inorganic Chemistry and the Periodic Table
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.