Explanation:

Ionic radii is the distance between the nucleus of an ion and its outermost electron. The size of an ion plays a crucial role in determining its properties. In a crystal structure, two ions can replace each other only if their ionic radii are similar in size.

Why 15%?

The percentage difference between ionic radii is an important factor in determining whether or not two ions can replace each other in a crystal structure. It has been observed that if the difference between the ionic radii of two ions is more than 15%, they cannot replace each other in a crystal structure.

Reason behind 15%

This is because if the difference between the ionic radii is too large, the crystal structure will be distorted, and the lattice energy will be affected. The lattice energy is the energy released when ions come together to form a crystal structure. If the lattice energy is affected, it can lead to instability in the crystal structure, making it less likely for two ions to replace each other.

Example:

For example, consider the crystal structure of sodium chloride (NaCl). Na+ and Cl- ions have ionic radii of 0.95 Å and 1.81 Å, respectively. The percentage difference between their ionic radii is (1.81 - 0.95)/1.81 x 100% = 47%. This percentage difference is too large for the Cl- ion to replace the Na+ ion in the crystal structure of NaCl.

Conclusion:

In conclusion, the difference between the ionic radii of two ions is an important factor in determining whether or not they can replace each other in a crystal structure. If the difference in ionic radii is more than 15%, the crystal structure will be distorted, and the lattice energy will be affected.