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Distance between body centered atom and a corner atom in sodium (a=4.225 A) is

Asked in CBSE AIPMT 1995 · Unit cells and lattice parameters

• (1) 2.99 A
• (2) 2.54 A
• (3) 3.66 A
• (4) 3.17 A

Step-by-step solution

Why the other options are wrong

• (1) 2.99 A is a/(√2), the FACE diagonal half-length, which is the nearest-neighbour distance in a face-centred cell, not a body-centred one.
• (2) 2.54 A matches no distance in this cell; working backwards it would need a=2.93 A.
• (4) 3.17 A is (3a)/4, which comes from squaring the √3 away and then halving twice.

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