Explain why the first ionisation energy of sodium is endothermic and why it is much lower in magnitude than the first ionisation energy of chlorine.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Ionisation energy is endothermic because energy must be supplied to overcome the electrostatic attraction between the nucleus and the outer electron.
Sodium has a larger atomic radius than chlorine, so its outer electron is further from the nucleus and experiences less attraction.
Sodium’s outer electron is shielded by more inner‑shell electrons, reducing the effective nuclear charge it feels.
Chlorine has a higher nuclear charge (17 protons vs 11) with similar shielding, giving a greater effective nuclear charge and a stronger attraction on its outer electron.
Sodium has a larger atomic radius than chlorine, so its outer electron is further from the nucleus and experiences less attraction.
Sodium’s outer electron is shielded by more inner‑shell electrons, reducing the effective nuclear charge it feels.
Chlorine has a higher nuclear charge (17 protons vs 11) with similar shielding, giving a greater effective nuclear charge and a stronger attraction on its outer electron.
Examiner tips
- Use the term ‘endothermic’ and explain energy input to break attraction.
- Mention atomic radius and shielding to justify lower attraction for Na.
- Compare nuclear charge and effective nuclear charge to explain higher IE for Cl.
Common mistakes
- Saying the ionisation energy is exothermic; it is actually endothermic.
- Confusing ionisation energy with electron affinity; they are different processes.
- Forgetting to mention the role of effective nuclear charge or shielding.
Mark scheme (4 marks)
- Ionisation energy is endothermic because energy must be supplied to overcome the electrostatic attraction between the nucleus and the outer electron.
- Sodium has a larger atomic/ionic radius than chlorine, so the outer electron is further from the nucleus and experiences less attraction.
- Sodium has greater shielding/screening of the outer electron by inner electron shells compared to the effective nuclear charge experienced by chlorine's outer electron.
- Chlorine has a higher nuclear charge (17 protons vs 11 in sodium) with a similar level of inner-shell shielding, resulting in a greater effective nuclear charge and therefore stronger attraction on the outer electron.
Key terms in this question
first ionisation energy · endothermic
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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