Which statement differentiates vacancy diffusion from interstitial diffusion and notes which is typically faster?

• A. Vacancy diffusion involves atoms moving through interstitial channels; interstitial diffusion uses lattice vacancies; interstitial diffusion is slower.
• B. Vacancy diffusion uses atoms hopping into vacant lattice sites; interstitial diffusion involves small atoms moving through interstitial sites; interstitial diffusion is typically faster.
• C. Both diffusion mechanisms are identical; speed depends on temperature only.
• D. Vacancy diffusion is faster than interstitial diffusion.

Answer: (B)

The essential idea is how atoms migrate through a crystal: one path requires an empty lattice site for an atom to hop into, while the other lets small atoms move through the gaps between lattice atoms. In vacancy diffusion, an atom moves by hopping into a neighboring vacant site; the vacancy effectively migrates as the atom moves. In interstitial diffusion, much smaller atoms navigate through interstitial sites between the regular lattice atoms, hopping from one site to another without displacing a host atom.

This distinction matters because the energy barrier for each path is different. Interstitial diffusion generally has a lower activation energy, since the migrating atom is small and moves through relatively open spaces without pushing on the surrounding lattice as much. The result is that interstitial diffusion is typically much faster than vacancy diffusion.

So, the statement correctly describes the two mechanisms and correctly notes that interstitial diffusion is typically faster. Other options mix up the mechanisms or claim equality or reversal of speed, which does not align with how these diffusion processes operate.