OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 21
13 marks · Medium difficulty · Structured Questions
Answer questions on atomic structure, isotopes, electron configurations, dot-and-cross diagrams, equations, and ionic lattices involving calcium, nitrogen, and oxygen.
Practise this questionQuestion
Question text
21 This question is about atomic structure and the compounds of calcium, nitrogen and oxygen.
(a) Most elements contain different isotopes.
State two differences between isotopes of the same element.
… [1]
(b) Complete the table for an atom and an ion of two different elements.
Mass
Element Protons Neutrons Electrons Charge
number
26 28 0
80 36 2−
[2]
(c) Electrons occupy orbitals which are arranged in energy levels.
In the diagram below, each box represents an orbital and each electron is shown as an arrow.
Label the sub-shells and add arrows to show how electrons occupy orbitals in an atom of
oxygen.
energy …
… 1s
[2]
(d) Calcium reacts with nitrogen to form calcium nitride, Ca3N2, which is an ionic compound.
(i) Construct a ‘dot-and-cross’ diagram for Ca3N2.
Show outer electrons only and the charges on each ion.
[2]
(ii) Calcium nitride reacts with water to form a solution containing two alkaline compounds.
Write an equation for this reaction.
… [2]
(iii) Calcium reacts with oxygen to form a compound which has a giant ionic lattice structure.
The diagram shows ions as circles in part of the lattice.
Complete the diagram by showing the symbols of the ions, including charges.
[2]
(iv) Nitrogen forms an oxide with the formula N2O. A molecule of N2O is linear and has a
nitrogen atom in the centre.
Draw a ‘dot-and-cross’ diagram for an N2O molecule.
Show outer electrons only.
[2]
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
21 (a) TWO correct responses from 1 AO1.1 IGNORE heavier/lighter
• Different numbers of neutrons
• Different (atomic) masses/mass numbers DO NOT ALLOW different relative atomic masses
BUT ALLOW different relative isotopic masses
• Different physical properties
Physical required DO NOT ALLOW different chemical properties
OR different properties
IGNORE different abundancies
(b) 2 AO1.2
Element Mass Protons Neutrons Electrons Charge ×2
number
THREE responses for each mark
Fe 54 26 28 26 0 Easiest to check element first
Se 80 34 46 36 2– 2–
ALLOW Se
ALLOW names for elements
Mark by row
(c) Sub-shells labels 2
2s (single box) AND 2p (3 boxes) AO1.1
Electrons as arrows
unpaired electrons in 3 boxes: ↑↓ ↑ ↑
AND
Paired electrons in single box: ↑↓ AO1.2
ALLOW single headed arrows, e.g. ↿
AO
element
(d) (i) 2 CARE:
ALLOW any pairing if electrons correct, e.g.
Ca shown with either 0 or 8 electrons
AND
N shown with 8 electrons with 5 dots and 3 crosses
AO2.5
(or vice versa) IF 8 electrons shown around Ca,
‘extra’ 3 electrons around N must match symbol for
3 Ca AND 2 N AND correct charges on ions, Ca electrons, e.g.
i.e. 3Ca2+ 2N3–
AO1.2
Circles OR Brackets NOT required
IGNORE inner shells
ALLOW drawing with 3 Ca2+ and 2 N3–
e.g.
(d) (ii) Ca3N2 + 6H2O → 3Ca(OH)2 + 2NH3 2 AO2.6 ALLOW NH4OH for NH3
×2
Ca(OH)2 OR NH3 as product ALLOW Ca3N2 + 8H2O → 3Ca(OH)2 + 2NH4OH
All species correct AND correct balancing IGNORE other products
AO
Question Answer 5 Marks Guidance
element
(d) (iii) 2 AO1.1 ALLOW labels if seen outside circles provided it
×2 clear which circle the label applies to
ALLOW 1 mark for Ca AND O shown alternately,
each in FOUR circles
i.e. with no charges or incorrect charges
ALLOW 1 mark for 2+/+2 AND 2–/–2 shown
Ca2+ shown alternately in FOUR circles alternately in FOUR circles (with no Ca and O)
O2– shown alternately in FOUR circles DO NOT ALLOW All circles with same ion,
i.e. all Ca2+ OR all O2–
ALLOW 1 mark for 4 Ca2+ AND 4O2– but NOT
shown alternately
e.g.
AO
element
(d) (iv) ‘Dot and cross’ of central N to O OR N 2 AO2.5 Electrons do NOT need to be shown paired.
×2
N → O N = O ‘Dot and cross’ of NO2
ALLOW 1st mark for N → O OR N = O
OR DO NOT ALLOW ions
N ≡ N N = N
CARE For 2nd mark, watch for stray paired OR
unpaired electrons on central N
OR OR
-------------------------------------------------------------------------
ALLOW 10 electrons around central N atom for
Rest of ‘dot and cross‘ diagram correct 2 marks,
i.e.
e.g. N ≡ N → O N = N = O
OR
Total 13
How to answer it
Atomic Structure and Compounds of Calcium, Nitrogen and Oxygen
What this question tests
This multi-topic question assesses core foundational concepts including isotope definitions, subatomic particle calculations, electron configurations (sub-shells and Hund's rule), dot-and-cross diagrams for ionic and covalent bonding, ionic lattice structures, and writing balanced equations for chemical reactions involving ionic nitrides.
Part (a): Isotopes
✅ Correct Answer
Any two of the following differences:
- Different numbers of neutrons
- Different (atomic) masses / mass numbers
- Different physical properties
❌ Common Errors
Students often lose this mark by stating "different relative atomic masses" (incorrect, as R.A.M. is a weighted average for an element) or claiming isotopes have different chemical properties (they have the same chemical properties because they have the same electron configuration).
Part (b): Subatomic Particles & Ions
✅ Correct Answers
Row 1 (Iron): Element = Fe , Mass number = 54 , Electrons = 26
Row 2 (Selenium): Element = Se , Protons = 34 , Neutrons = 46
💡 Key Knowledge
Remember that for a neutral atom, protons equals electrons. For an ion, the charge tells you the difference: Electrons = Protons − Charge . For Se²⁻ with a 2− charge and 36 electrons, protons = 36 − 2 = 34.
Part (c): Electron Configurations
✅ Correct Answer
Sub-shells: Label the next box 2s and the group of three boxes 2p .
Arrows: Single box for 2s gets a paired set of arrows (↑↓). The three 2p boxes get single unpaired electrons first (three parallel arrows pointing up: ↑ ↑ ↑), followed by one paired electron in the first 2p box (↑↓).
🧠 Exam Technique
Apply Hund's rule: electrons fill degenerate orbitals singly before pairing up, with parallel spins. Oxygen has atomic number 8 (electron configuration 1s² 2s² 2p⁴).
Part (d)(i): Ionic Dot-and-Cross Diagram (Ca₃N₂)
✅ Correct Answer
Show 3 calcium ions: 3[Ca]²⁺ (each with 0 or 8 outer electrons).
Show 2 nitrogen ions: each with a 3− charge, surrounded by 8 outer electrons (e.g., a mix of dots and crosses showing transfer).
❌ Common Errors
Forgetting the stoichiometric coefficients (the large 3 in front of Ca²⁺ and 2 in front of N³⁻) will cost you a mark, as the formula ratio must be clearly represented.
Part (d)(ii): Equation for Calcium Nitride and Water
✅ Correct Answer
Ca₃N₂ + 6H₂O → 3Ca(OH)₂ + 2NH₃
💡 Key Knowledge
Calcium nitride reacts with water to produce calcium hydroxide (an alkali) and ammonia gas ( NH₃ ), both of which are alkaline compounds satisfying the stem of the question.
Part (d)(iii): Ionic Lattice Diagram
✅ Correct Answer
Complete the diagram by labeling the alternating circles with ion symbols and charges: Ca²⁺ and O²⁻ arranged alternately across the four interconnected unit cells.
🧠 Exam Technique
Ensure that every adjacent ion is of the opposite charge to correctly depict the alternating giant ionic lattice structure.
Part (d)(iv): Covalent Dot-and-Cross Diagram (N₂O)
✅ Correct Answer
A linear molecule with a central nitrogen atom bonded to an oxygen atom on one side and a nitrogen atom on the other ( N = N = O or N → N = O structures), showing correct sharing of outer shell electrons to give octets.
❌ Common Errors
Watch out for stray unpaired electrons or incorrect placement on the central nitrogen atom. Examiners penalize structures where octet rules are violated without valid coordinate bonding representations.
Topics
Module 2: Foundations in chemistry · 2.1 Atoms and reactions · 2.2 Electrons, bonding and structure
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.