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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.

1 Answer

(a) Similarities:
(i) Both involve electrostatic forces of attraction
(ii) Both are non-directional.
Differences: Ionic bond is a strong bond due to electrostatic forces of attraction while metallic bond may be weak or strong depending upon the kernels.
(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.
(c)Yes because its repetition in different directions produces the complete space lattice.
(d) Ionic solids are hard because there are strong electrostatic forces of attraction. However they are brittle because the bond is non-directional.
answered Jun 3, 2014 by sreemathi.v

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