Consider two systems: A piece of crystalline NaCl (page 562 figure 12.1B) and a

Question

Consider two systems: A piece of crystalline NaCl (page 562 figure 12.1B) and a piece of solid gold (Figure 12.2A). In an ionic crystal such as crystalline NaCl, sodium and chloride ions occupy discrete lattice points. Electrons have been transferred from the sodium atoms to the chloride ions and so are very localized. In the gold sample, the gold nuclei occupy lattice points but are surrounded by a delocalized cloud of valence electrons. If you hit a sample of gold with an iron hammer the sample distorts (it is malleable) but does not break. If you hit an ionic crystal of NaCl with a hammer it will shatter (break). Why does the metal distort and the ionic crystal break? Use Coulombs law type thinking.

Explanation / Answer

According to coulomb's law, there is a force of attraction between oppositely charged species. So the strength of the crystalline NaCl depends on the force of attraction between the sodium and chloride ions. When a crystal is hit and the bond is broken, the entire crystal loses its strength. So it shatters. A gold lattice has a declocalized cloud of valence electrons.There is no individual attraction between the particles like in NaCl. So when force is applied, the entire block or crystal deforms. So gold is malleable while the ionic crystal is brittle.

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