Interstitial diffusion is associated with impurities such as hydrogen, carbon, nitrogen, and oxygen because their atoms are small enough to fit into interstitial positions.

• A. Copper and nickel
• B. Silicon and germanium
• C. Hydrogen, carbon, nitrogen, and oxygen
• D. Aluminum and magnesium

Answer: (C)

Interstitial diffusion relies on impurity atoms being small enough to fit into the gaps between host atoms in a crystal and then hop from one interstitial site to another. Hydrogen, carbon, nitrogen, and oxygen are classic examples because their atomic sizes are much smaller than the metal atoms forming the lattice, so they can occupy octahedral or tetrahedral interstitial sites without pushing the lattice apart. This enables relatively rapid movement through the lattice by repeated hopping, which is why these elements are commonly associated with interstitial diffusion in metals.

The other elements listed are not typically able to occupy interstitial sites in the same way. Copper, nickel, aluminum, and magnesium are metal atoms similar in size to the host lattice atoms, so diffusion for them is usually substitutional or vacancy-mediated rather than interstitial. Silicon and germanium form covalent networks with different diffusion behavior, not the classic interstitial diffusion seen for the small impurity elements.