Self-diffusion occurs when diffusing atoms are the same type.

• A. Atoms of different types diffuse
• B. Atoms diffuse only from a surface
• C. Diffusion cannot occur in pure metals
• D. The same type of atoms diffuse

Answer: (D)

Self-diffusion is diffusion where the diffusing species are identical to the host lattice atoms. In a pure metal, atoms move by jumping to neighboring vacant lattice sites (or through interstitial positions) without introducing a different element into the lattice, so the overall composition remains the same even as atoms exchange places. This contrasts with interdiffusion, where atoms of different types diffuse into each other and change local composition. Diffusion can occur in the bulk, not just at surfaces, through vacancy or interstitial mechanisms. So the statement that the same type of atoms diffuse accurately captures what self-diffusion means.