Edexcel A-Level Chemistry Paper 1, June 2019: Question 1

Atoms, Molecules and Ions (Edexcel A-Level Chemistry)

What this question tests

This foundational multiple-choice question set assesses core atomic structure and subatomic particle calculations, mass spectrometry fragmentation patterns for diatomic molecules with isotopes, electronic configurations of ions in terms of subshell orbitals, and periodic trends in ionic radius across isoelectronic series.

Part (a)

Subatomic Particles in an Oxygen-18 Atom

✅ Correct Answer: A

8 protons, 10 neutrons and 8 electrons

Awarded 1 mark for identifying the correct combination.

💡 Key Knowledge

Atomic number (proton number) of oxygen is 8, meaning it always has 8 protons.
In a neutral atom, number of electrons equals the number of protons (8 electrons).
Neutrons = Mass Number (18) minus Atomic Number (8) = 10 neutrons.

❌ Common Errors

Options B, C, and D are incorrect because oxygen atoms always possess 8 protons. Changing the proton number changes the element entirely.

Part (b)

Mass Spectrum of Bromine Molecules

✅ Correct Answer: D

Reflects three sets of molecular ion peaks corresponding to 79Br2 , 79Br-81Br , and 81Br2 in a 1:2:1 ratio due to roughly equal isotopic abundance.

Awarded 1 mark for selecting spectrum D.

🧠 Exam Technique

Bromine exists as a diatomic molecule ( Br₂ ) with two main isotopes: ⁷⁹Br and ⁸¹Br in a 1:1 ratio.
Molecular ion peaks ( m/z ) form at:
• 79 + 79 = 158 (Height 1)
• 79 + 81 = 160 (Height 2, because it can form two ways: ⁷⁹Br-⁸¹Br or ⁸¹Br-⁷⁹Br)
• 81 + 81 = 162 (Height 1)

❌ Common Errors

A has no peaks at the molecular mass range (158-162). B misses the mixed isotopic peak at m/z 160. C incorrectly shows a 1:1:1 ratio instead of accounting for statistical probability (1:2:1).

Part (c)

Electrons in Occupied p Orbitals of a Chloride Ion

✅ Correct Answer: C

12 electrons in occupied p orbitals.

Awarded 1 mark for selecting C.

💡 Key Knowledge

Chloride ion ( Cl⁻ ) has an electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ .
Occupied p orbitals include 2p (6 electrons) and 3p (6 electrons).
Total p-electrons = 6 + 6 = 12.

❌ Common Errors

A (5) and B (6) fail to account for all occupied p subshells across different principal energy levels. D (18) represents the total number of electrons in the chloride ion, not just those in p orbitals.

Part (d)

Ionic Radius in an Isoelectronic Series

✅ Correct Answer: A

N³⁻ has the largest ionic radius.

Awarded 1 mark for selecting A.

🧠 Exam Technique

All listed ions ( N³⁻ , O²⁻ , Na⁺ , Al³⁺ ) are isoelectronic—they all have the exact same electron configuration (10 electrons, like Neon).
When electronic structure is constant, ionic radius is governed entirely by nuclear charge (proton number).
As nuclear charge increases, the inward electrostatic pull from the nucleus on the electron cloud increases, pulling electrons closer and shrinking the radius.

❌ Common Errors

Students often confuse isoelectronic species with atomic radius trends. Aluminium ( Al³⁺ , 13 protons) exerts the strongest nuclear attraction, resulting in the smallest radius, whereas nitrogen ( N³⁻ , 7 protons) has the weakest nuclear pull, resulting in the largest radius.

Edexcel A-Level Chemistry Paper 1, June 2019: Question 1

Question

Mark scheme

How to answer it

Atoms, Molecules and Ions (Edexcel A-Level Chemistry)

What this question tests

Subatomic Particles in an Oxygen-18 Atom

✅ Correct Answer: A

💡 Key Knowledge

❌ Common Errors

Mass Spectrum of Bromine Molecules

✅ Correct Answer: D

🧠 Exam Technique

❌ Common Errors

Electrons in Occupied p Orbitals of a Chloride Ion

✅ Correct Answer: C

💡 Key Knowledge

❌ Common Errors

Ionic Radius in an Isoelectronic Series

✅ Correct Answer: A

🧠 Exam Technique

❌ Common Errors

Topics