Explain how much portion of an atom located at $(i)$ corner and $(ii)$ body centre of a cubic unit cell is part of its neighbouring unit cell.

(N/A) $(i)$ An atom located at the corner of a cubic unit cell is shared by $8$ adjacent unit cells. Therefore,$\frac{1}{8}$ portion of the atom belongs to one unit cell.
$(ii)$ An atom located at the body centre of a cubic unit cell is not shared by any neighbouring unit cell. Therefore,the atom belongs entirely to the unit cell in which it is present,i.e.,its contribution to the unit cell is $1$.