The number of atoms is calculated as: For 0.5mol=0.5×6.022×1023 =3.011×1023 Tetrahedral voids is equal to 2 × Number of atoms of metal Octahedral voids is equal to number of atoms of metal Therefore, Number of tetrahedral voids =2×3.011×1023 =6×1023 =1mol Number of octahedral voids =3.011×1023 =0.5mol The total number of voids is 1+0.5=0.5mol