Ionic structures and bonding
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Why is a crystal of rock salt much harder than a sugar crystal?
Rock salt shatters when struck while a covalent compound tends to be soft and easily crushed. Why?
Igloo writes ...
Your questions have much to do with the different types of bonding shown by ionic and covalent compounds.
Ionic compounds such as rock salt (sodium chloride) consist of a three-dimensional network of ions held together by relatively strong electrostatic forces of attraction, so that a great deal of energy is required to break them, which is what happens when the crystal is melted or, in your case, shattered when struck.
In contrast covalent compounds consist of molecules, not ions, and these are held together by much weaker forces than ionic ones – van der Waals, dipole-dipole or hydrogen bonding – and so less energy is required to prise them apart during melting, for example. The softness of covalent compounds reflects the fact that these attractive forces are much weaker than those which apply to ionic compounds.
Candle wax (a mixture of alkanes) consists of molecules held together by van der Waals forces so the wax is soft and melts between about 50 °C and 80 °C.
Sugar is an interesting substance in that although it is covalent, there is extensive hydrogen bonding present between the molecules, and whilst the forces of attraction operating are not as great as they would be if they were ionic, they are much stronger than the van der Waals forces of attraction in candle wax. This is why sugar is more crystalline and the crystals are more brittle than those of the wax. However, owing to the ionic bonding present, crystals of sodium chloride have even greater “hardness” and “brittleness” than those of sugar.
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updated: 22 August 2003
