Molecular crystals such as solid iodine have low melting points because
Correct answer: C. only weak intermolecular forces have to be overcome on melting
- A. the covalent bonds within the molecules are weak
- B. they contain no bonds at all
- C. only weak intermolecular forces have to be overcome on melting
- D. their molecules are very heavy
Explanation
Melting separates molecules from one another, and in a molecular crystal they are held only by dispersion forces or dipole interactions, which are far weaker than ionic or covalent bonds. The strong covalent bonds inside each iodine molecule are not broken at all when the solid melts, which is the distinction the question turns on. This is also why such solids are soft and often volatile.
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About Ionic and Molecular Crystals
Ionic crystals contain oppositely charged ions held by electrostatic forces, whereas molecular crystals contain discrete molecules joined by weaker intermolecular forces. Their structures explain differences in melting point, hardness, volatility, solubility and electrical conductivity, especially between solid and molten states.
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