Answer is option 1. How to solve?
In NACL Na occupies all the octahedral voids which is present at edge centre and one on the body centre...therefore total Na+ ions=12+1=13
@Aditya-Dubey there was a correction in the answer key. now the answer is given option 3. Is this correct?
In the above illustration we can observe that Cl- ions are forming the cubic close packed structure whereas the Na+ ions are occupying the octahedral voids
Cl- ---->corners and face centers
Na+ ----->edge centers and body centre
effective number of sodium ions
12(1/4) +1 =3+1=4
effective number of Cl- ions
6(1/2)+ 8(1/8)= 4
Formula of the unit cell -(NaCl)4
there are 4 molecules of NaCl in one unit cell and hence 4 sodium ions ...hence the given option is perfectly right