Question

Which of the following defects is observed in steel?

• A. Substitutional impurity defect
• B. Interstitial impurity defect
• C. Vacancy defect
• D. Schottky defect

Hint:

Carbon atoms sit in the gaps between iron atoms in steel.

Answer 1

The Correct Option is B

Step 1: Approach
Think about the sizes of atoms. Iron has a radius of about 126 pm and carbon about 77 pm.

Step 2: Fit
A carbon atom is small enough to slip into the octahedral gaps of iron. It does not replace an iron atom, and no lattice site is left empty.

Step 3: Name of the defect
A foreign atom lodged in a gap is an interstitial impurity. This is why steel is harder than pure iron: the carbon atoms block the slipping of layers.

Step 4: Conclusion
Substitutional defects need similar-sized atoms, and vacancy and Schottky defects involve missing atoms or ions. So the correct option is (B).

Final Answer:
Carbon atoms occupy gaps in the iron lattice, an interstitial impurity defect, option (B). \[ \boxed{\text{Interstitial impurity defect}} \]