CBSE Class 12-science Answered

According to the structure of NaCl 
Here, 'A'  will occupy 8 corners of  cubic unit cell which will contribute 1/8 each to the cube
therefore 8*1/8 = 1
and 6 face centered atoms will contribute 1/2 to the cube
hence 6*1/2=3
Therefore total contribution of atom'A' is  3+1=4
Now B occupies octahedral voids (NaCl structure)  which contributes 1/4
therefore 1+12*1/4 = 4
So the formula is A A4B4 or AB
Now  we are removing all  face centered atoms along one axis
so the number of atoms removed from cube are 2
so contribution from 'A' remains 8*1/8(corner atoms) + 4*1/2(facecentered atoms) = 3
so the resultant becomes A3B4