Radius ratio Chemistry Notes
The ratio between radii of cation and anion is called radius ratio.
- Radius ratio = \(\frac{\text { Radius of Cation }}{\text { Radius of Anion }}\)
- Radius ratio = \(\frac{r^{+}}{r^{-}}\)
→ We can get information about coordination number of cation or anion in ionic crystals with the help of radius ratio. So, the structure of ionic crystal lattice can also be determined by radius ratio. It is called radius ratio law. Different coordination number and radius ratio of structure of crystal lattice is given in following table 1.5.
Limiting Radius Ratio and Crystalline Structure
| Limiting | Radius Ratio 1 | Coordination
Number |
Structure of | Crystal | Example |
| 0.155 – 0.225 | 3 | Trigonal planar | B2O3 |
| 0.225 – 0.414 | 4 | Tetrahedral | ZnS |
| 0.414 – 0.732 | 6 | Octahedral i | NaCl, KCl |
| 0.732 – 1.00 | 8 | Cubic | CsCl, RbCl |