CHEMISTRY · Single correct · A Level · Chemistry · Bonding · Bonding
Problem
In a shear test, a bar of NaCl and a bar of copper, both cut to identical dimensions, are subjected to equal lateral stress. The copper sample bends, whereas the NaCl sample fractures along a plane. Which of the following best explains the different responses at the atomic level?
(a) Copper’s face-centered cubic structure contains many slip systems that make the metal ductile, whereas NaCl has a body-centered cubic lattice with very few slip systems, so it is brittle.
(b) Copper atoms are bonded by nondirectional metallic interactions that allow planes to slip while the delocalized electrons maintain cohesion, but in NaCl a slip would place like charges adjacent, producing strong repulsion and brittle fracture.
(c) Copper has a lower melting point than NaCl, so it softens under the applied stress, allowing deformation, whereas NaCl remains solid and therefore breaks.
(d) Because copper consists of neutral atoms, sliding one layer over another does not introduce new electrostatic forces, but in NaCl strong attractions between unlike ions lock the layers in place, so the lattice breaks when stress is applied.