Question

A solid ab has nacl type structure in which a atoms occupy the corners of the cubic unit cell. If all the face centred atoms along one of the axes are removed, the resulting stoichiometry will be

Answer 1

According to NaCl structure

 'A' occupies 8 corners of the cubic unit cell which contribute 1/8 each to this cube

therefore 8*1/8 = 1

also 6 face centered atoms contribute 1/2 to this cube

therefore 6*1/2=3

Therefore total contribution of atom'A' is 4

Now B occupies octahedral voids(NaCl structure) which contribute 1/4

therefore 1+12*1/4 = 4

So the formula is A4B4 or AB

Now if we are removing all the face centered atoms along one axis(any axis origin at centre of cube)

so the number of atoms removed from cube are 2

so contribution from 'A' remains 8*1/8(corner atoms) + 4*1/2(face centered atoms) = 3

so the resultant becomes A3B4