A rock-salt type unit cell has cations M at the corners and face centres and anions X at the edge centres and the body centre. All ions lying on one body diagonal (the two corner cations and the central anion) are removed. Find the ratio (number of M)/(number of X) per remaining unit cell (two decimals).

Numerical answer type, tolerance 0.01.

Answer: 1.25

Full cell: M

= 8 \times \frac{1}{8} + 6 \times \frac{1}{2} = 4

; X

= 12 \times \frac{1}{4} + 1 = 4

. Removing two corner M removes

2 \times \frac{1}{8} = 0.25

of M, leaving 3.75. Removing the body-centre X removes 1, leaving 3. Ratio

= 3.75/3 = 1.25