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

✅ Correct Answer

• Similarity: Both isotopes have 35 protons.
• Difference: ⁷⁹ Br has 44 neutrons, and ⁸¹ Br has 46 neutrons. (Or state that one has 2 more neutrons than the other).

💡 Key Knowledge

Isotopes are atoms of the same element with the same atomic number (protons) but different mass numbers due to a different number of neutrons.

🧠 Exam Technique

Read the stem carefully: the question explicitly asks to refer to the number of particles in the nuclei. Simply saying "they have different mass numbers" will not score the mark.

❌ Common Errors

Referring to electrons instead of protons/neutrons. The question specifically restricts your answer to particles in the nuclei.

✅ Correct Answer

They have the same electronic configuration (or the same number of electrons / 35 electrons).

💡 Key Knowledge

Chemical reactions involve interactions between electrons (specifically outer-shell electrons). Since isotopes share the exact same electron arrangement, their chemical properties are identical.

🧠 Exam Technique

Keep your answer concise and focused entirely on electrons. Avoid vague references to "outer shells only" unless tied directly to the full electronic configuration.

❌ Common Errors

Stating that they have the "same number of electrons in their outermost shell" without linking it to the overall electronic structure can sometimes be too vague for examiners.

✅ Correct Answer

Use a mass spectrometer to compare the intensity of the signal / number of particles for each isotope detected.

💡 Key Knowledge

Mass spectrometry separates ions according to their mass-to-charge ratio (m/z) and measures their relative abundances from the height of the detector peaks.

🧠 Exam Technique

You must explicitly name the analytical instrument (mass spectrometer) and state what is measured (signal intensity or relative abundance of ions).

❌ Common Errors

Suggesting general chemical or physical separation techniques like filtration, titration, or fractional distillation, which cannot separate isotopes.

✅ Correct Answer

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵

(Note: 3d¹⁰ 4s² in either order is fully accepted).

💡 Key Knowledge

Bromine has an atomic number of 35. Following Aufbau filling principles up to the 4p orbital gives a total of 35 electrons distributed across the subshells.

🧠 Exam Technique

Always double-check that your superscript numbers add up to the atomic number of the element (2 + 2 + 6 + 2 + 6 + 2 + 10 + 5 = 35).

❌ Common Errors

Miscounting electrons in the p-subshells (e.g., writing 4p⁶ instead of 4p⁵ , which would represent Krypton).

✅ Correct Answer

B: 7

💡 Key Knowledge

The fourth quantum shell (principal quantum number n = 4) includes all orbitals starting with 4. For bromine, this is 4s² 4p⁵ , giving a total of 2 + 5 = 7 electrons.

🧠 Exam Technique

Scan the electronic configuration for all terms where the principal quantum number (the big number) is 4: here, 4s² and 4p⁵ . Sum their superscripts.

❌ Common Errors

Choosing option D (18) by confusing the third quantum shell capacity, or choosing option C by mistakenly including the 3d subshell in the fourth shell.