How many effective `Na^(+)` ions are present in the rock salt NaCl if ions along one axis joining opposite faces are removed? 

Answer

3

Explanation

`Na^(+)` at edge centres present in the opposite faces are removed `4xx(1)/(4)=1`. Remaining `Na^(+)` at the edge centres `8xx(1)/(4)=2` and at the body centre `1xx1=1`, Total `=2+1=3`