Dentists historically shaped pure gold into thin sheets and pressed them into…
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
Dentists historically shaped pure gold into thin sheets and pressed them into cavities because the metal can be cold-welded and shaped easily. Modern dental crowns, however, are often made from an alloy that is 95 % gold and 5 % copper by mass.
The atomic radius of Cu (128 pm) is slightly smaller than that of Au (144 pm). Assume the alloy has a random distribution of Cu atoms within the gold lattice.
(a) Identify whether this alloy is best described as a substitutional or an interstitial alloy and justify your choice with reference to relative atomic sizes.
(b) Explain qualitatively how the presence of copper atoms affects the movement of dislocations (slip planes) within the gold lattice.
(c) Predict how the malleability and hardness of the alloy compare with those of pure gold. Provide particle-level reasoning to support your prediction.
(d) Discuss whether the electrical conductivity of the 95 % Au–5 % Cu alloy is expected to be higher, lower, or approximately the same as that of pure gold. Justify your answer in terms of electron mobility and lattice imperfections.