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Practice · Chemistry · 2025 syllabus · S1.2

IB Chemistry S1.2 The nuclear atom: practice questions

6 exam-style questions on S1.2 The nuclear atom, SL and HL, 3 marked HL: 4 multiple choice and 2 structured. Each one comes with the mark scheme, a model answer and the reasoning behind every mark. Free with your email.

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S1.2-01 · Paper 1A style · multiple choice · 1 mark

Particles in a negative ion

How many protons, neutrons and electrons are present in one 79Se2− ion? [1]

  1. 79 protons, 45 neutrons, 36 electrons
  2. 34 protons, 45 neutrons, 32 electrons
  3. 34 protons, 45 neutrons, 36 electrons
  4. 34 protons, 79 neutrons, 36 electrons
Show the answer and the reasoning

Answer: C. C is correct. Selenium has atomic number 34, so every selenium atom or ion has 34 protons. Neutrons = mass number − protons = 79 − 34 = 45. A 2− charge means two electrons more than protons, so 34 + 2 = 36 electrons. A uses the mass number as the number of protons. B treats the 2− charge as two electrons lost; a negative ion has gained electrons. D uses the mass number as the number of neutrons, forgetting that the mass number counts protons and neutrons together. Find Z in the data booklet first, then work out neutrons from A − Z and electrons from the charge.

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The 6 questions in S1.2

  1. S1.2-01 Particles in a negative ionPaper 1A style · multiple choice · 1 mark
  2. S1.2-02 Isotopes and physical propertiesPaper 1A style · multiple choice · 1 mark
  3. S1.2-03 Relative atomic mass from peaksPaper 1A style · multiple choice · 1 mark · HL
  4. S1.2-04 Molecular peaks of a diatomic elementPaper 1A style · multiple choice · 1 mark · HL
  5. S1.2-05 Gallium isotopes and abundancePaper 2 style · structured question · 5 marks
  6. S1.2-06 Mass spectrum of chlorine gasPaper 2 style · structured question · 6 marks · HL

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Every Biology and Chemistry question here is original, written for practice in the style of the papers set since the 2025 syllabus, with invented data. Developed independently from the International Baccalaureate Organization, which has not endorsed it. Found a mistake? Email vytis@limeadmissions.com with the question id.