GATE 2020 MT – Question 24
The angle between neighbouring tetrahedral bonds in diamond-cubic silicon is
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Correct answer: (B) 109.5°
Explanation
Choose two tetrahedral bond directions proportional to (1,1,1) and (1,−1,−1). Their dot product is −1 and each has length √3, giving $\cos\theta=-1/3$. Thus θ = 109.47°, approximately 109.5°, B.