Free Ionic and Molecular Crystals MCQs with Answers

1. Ionic crystals such as sodium chloride are hard and have high melting points because

• A. their ions are held by weak van der Waals forces
• B. strong electrostatic forces act between oppositely charged ions throughout the lattice
• C. they contain covalent bonds between the ions
• D. the ions are free to move at room temperature

Explanation: Every ion attracts all the oppositely charged ions around it, and a great deal of energy is needed to break this extended three dimensional network, so the melting point of sodium chloride is 801 degrees Celsius. The ions are locked in place in the solid, which is why solid sodium chloride does not conduct electricity while the molten salt and its solution do.

2. Molecular crystals such as solid iodine have low melting points because

• 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.

3. Which of the following is a covalent network solid?

• A. Ice
• B. Sodium chloride
• C. Solid carbon dioxide
• D. Diamond

Explanation: In diamond every carbon is covalently bonded to four others in a continuous three dimensional network, so melting it means breaking covalent bonds and the melting point is above 3500 degrees Celsius. Ice and dry ice are molecular solids held by hydrogen bonds and dispersion forces respectively, and sodium chloride is ionic. Silicon dioxide and silicon carbide are the other standard network examples.

4. Solid sodium chloride does not conduct electricity, but molten sodium chloride does, because

• A. melting converts the ions into atoms
• B. the ions become free to move only when the lattice breaks down
• C. electrons are released on melting
• D. the molten salt contains water

Explanation: Conduction in an ionic compound needs mobile charge carriers, and in the solid the ions are locked in fixed lattice positions even though they are charged. Melting frees them to migrate towards the electrodes, and dissolving in water has the same effect. No electrons are released and the ions remain ions throughout, which rules out the other explanations.

5. Metals are malleable and ductile because

• A. their atoms are held in rigid directional covalent bonds
• B. they contain no free electrons
• C. layers of positive ions can slide over one another while the sea of delocalised electrons holds them together
• D. their ions are held by hydrogen bonds

Explanation: Metallic bonding is non directional, so displacing one layer of cations does not break the structure because the delocalised electrons continue to bind whatever arrangement results. An ionic crystal shatters instead, because sliding brings like charges opposite each other and the layers repel. The same free electrons account for electrical and thermal conductivity.

6. Which of the following solids is held together by dispersion forces only?

• A. Solid iodine
• B. Sodium chloride
• C. Diamond
• D. Copper

Explanation: Iodine molecules are non polar, so the only attraction between them is the temporary induced dipole interaction known as the dispersion force, which is why iodine sublimes readily on gentle warming. Sodium chloride is ionic, diamond covalent network and copper metallic, all of which involve far stronger bonding. Dispersion forces still grow with molecular size, which is why iodine is a solid while chlorine is a gas.