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Question:

The ionic radii of A⁺ and B⁻ ions are 0.98 × 10⁻¹⁰ m and 1.81 × 10⁻¹⁰ m. The coordination number of each ion in AB is:

8

2

4

6

Solution:

The radius ratio rA⁺/rB⁻ = 0.98 × 10⁻¹⁰ / 1.81 × 10⁻¹⁰ = 0.54

According to the radius ratio rule:

If radius ratio is between 0.225 and 0.414, the coordination number is 4.
If radius ratio is between 0.414 and 0.732, the coordination number is 6.
If radius ratio is between 0.732 and 1.000, the coordination number is 8.

Since the radius ratio (0.54) falls between 0.414 and 0.732, the coordination number of each ion in AB is 6.