Solid State
- The packing efficiency of a body-centred cubic (bcc) unit cell is Answer: C. 68% In bcc, 4r=sqrt3a ; two atoms occupy 2·4/3π r³/a³≈0.68 of the cell. 1
- An element with molar mass 60 g mol-1 forms an fcc lattice with edge length 400 pm. Its density is approximately (\(N_A=6.022\times10^{23}\)) Answer: D. 6.23 g cm-3 ρ=ZM/a³N_A=frac4×60(4×10⁻⁸)³×6.022×10²³=240/38.54≈6.23 g cm⁻³. 1
- In a cubic solid, atoms A occupy the corners and atoms B occupy the face centres. If one A atom is missing from one corner of each unit cell, the formula of the compound is Answer: D. A7B24 A =7×1/8=7/8 , B =6×1/2=3 . Ratio 7/8:3=7:24 . 1
- Which of the following defects does NOT change the density of an ionic crystal? Answer: D. Frenkel defect In a Frenkel defect an ion only shifts to an interstitial site, so mass and volume stay the same. 1