Q.How will you distinguish between the following pairs of terms:

$(i)$ Both are close packings with $74\%$ efficiency and coordination number $12$, but hcp has ABAB layer sequence while ccp has ABCABC.

$(ii)$ A crystal lattice is the whole regular 3-D array of points; a unit cell is its smallest repeating portion. …

(i) Hexagonal close-packing (hcp) vs cubic close-packing (ccp).

Both have packing efficiency $74\%$ and coordination number $12$. The difference is the stacking of layers:

• hcp: ABAB… (third layer directly over the first).
• ccp: ABCABC… (third layer over the tetrahedral voids of the first, i.e. a new position).

(ii) Crystal lattice vs unit cell.

• A crystal lattice is the regular three-dimensional arrangement of all the points (positions of constituent particles) in the crystal.
• A unit cell is the smallest repeating unit of the lattice which, on repetition in three dimensions, generates the whole lattice.

(iii) Tetrahedral void vs octahedral void.

Point-by-point contrast: the stacking sequence separates hcp/ccp; scale (whole vs smallest repeat) separates lattice/unit ce …

• Saying hcp and ccp have different packing efficiencies (both are $74\%$). …