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# Explain:(a)The basis of similarities and difference between metallic and ionic crystals.(b) Unit cell is not simply a cube of $4Na^+$ ions and $4Cl^−$ ions.(c) Can a cube consisting of $Na^+$ and $Cl^−$ ions at alternate corners serve as satisfactory unit cell for the sodium chloride lattice?(d) Ionic solids are hard and brittle.

(b) Unit cell of NaCl has fcc arrangement of $Cl^−$ ions and $Na^+$ ions are present at the edge centres and one at the body-centre.
Thus there are $14Cl^−$ ions and $13Na^+$ ions in the unit cell. However their net contribution towards the unit cell is $4Na^+$ and $4Cl^−$ ions.