Question

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Answer 1

Hey... Here is ur answer...



Suppose number of atoms Q in ccp arrangement $$=100$$
The number of tetrahedral sites $$=200$$
As all the tetrahedral sites are occupied by atoms P, their number $$=200$$
Hence, the ratio of $$P\,\colon\,Q=2\,\colon\,1$$.
Therefore, the formula is $$P2Q$$.



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Answer 2

Q atoms occupy ccp, so the atoms are present at 'face centres' as well as 'corners'.

There are 6 face centres in a unit cell each being shared by 2 unit cells;

→so we calculate the effective number of atoms as 6 x ¹/₂ = 3

There are 8 corners in a unit cell each being shared by 8 other unit cells;

→so, effective number of atoms = 8 x ¹/₈ = 1

Thus the effective number of Q atoms in a unit cell = 3 + 1 = 4

P atoms occupy all the tetrahedral voids,

There are 8 tetrahedral voids per unit cell, with 2 voids on each body diagonal.

→Effective number of P atoms per unit cell = 8

Thus the ratio of P:Q becomes 8:4 = 2:1

Hence the formula is P₂Q

If you cannot quite get the point, then you can refer the picture in the attachment below↓