Potassium iodide is an ionic compound formed when potassium reacts with iodine. Explain how potassium iodide is formed from its elements, and why the compound has a high melting point.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Potassium (K) is in Group 1 of the periodic table. Iodine (I) is in Group 7 of the periodic table. Potassium iodide (KI) is used in table salt and some medical treatments.
Model answer (5 marks)
Potassium loses one electron from its outermost s‑orbital to give a K⁺ ion.
Iodine gains one electron into its outer p‑orbital to give an I⁻ ion.
Both ions now have a full outer shell, achieving a noble‑gas configuration.
The K⁺ and I⁻ ions are held together by strong electrostatic attraction – an ionic bond.
In the crystal lattice of KI many such ionic bonds exist; a large amount of energy is required to break these bonds, giving the compound a high melting point.
Iodine gains one electron into its outer p‑orbital to give an I⁻ ion.
Both ions now have a full outer shell, achieving a noble‑gas configuration.
The K⁺ and I⁻ ions are held together by strong electrostatic attraction – an ionic bond.
In the crystal lattice of KI many such ionic bonds exist; a large amount of energy is required to break these bonds, giving the compound a high melting point.
Examiner tips
- Show the electron transfer step by step; use the symbols K⁺ and I⁻. Mention the noble‑gas configuration to justify stability. Explain that the lattice contains many ionic bonds, which is why the melting point is high.
- common_mistakes
- :
- Writing that iodine loses an electron instead of gains one. Forgetting to mention the noble‑gas configuration. Saying the compound has a high melting point because it is ionic without linking to the lattice energy.
Mark scheme (5 marks)
- Potassium loses one electron (from its outer shell) to form a K⁺ ion
- Iodine gains one electron (into its outer shell) to form an I⁻ ion
- Both ions achieve a full outer shell / stable electronic configuration (like a noble gas)
- There are strong electrostatic forces of attraction between the oppositely charged K⁺ and I⁻ ions (ionic bonds)
- A large amount of energy is needed to break the many strong electrostatic forces of attraction / ionic bonds throughout the giant ionic lattice, so the melting point is high
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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