Every atom or ion that forms an FCC unit cell is surrounded by:

Six octahedral voids and eight tetrahedral voids

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Eight octahedral voids and six tetrahedral voids

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Six octahedral voids and six tetrahedral voids

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Eight octahedral voids and four tetrahedral voids.

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Solution

The correct option is A Six octahedral voids and eight tetrahedral voids

Look at the corner atom of an FCC unit cell. A corner atom connects 8 unit cells. In each cube, at a distance of $\frac{\sqrt{3} a}{4}$ from the corner atom, there is a tetrahedral void; this implies each atom is surrounded by 8 tetrahedral voids. At the centre of edge, there is an octahedral void. Each corner atom is connected to 6 edges and hence, it is surrounded by 6 octahedral voids.

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Similar questions

In which of the following structures coordination number for cations and anions in the packed structure will be same ?

(a) $C l^{-}$ ions form fcc lattice and $N a^{+}$ ions occupy all octahedral voids of the unit cell

(b) $C a^{2 +}$ ions form fcc lattice and $F^{-}$ ions occupy all the eight tetrahedral voids of the unit cell

(d) $S^{2 -}$ ions form fcc lattice and $Z n^{2 +}$ ions go into alternate tetrahedral voids of the unit cell

A B

A^{+}

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