How does the radius ratio rule apply to ionic compounds?

The radius ratio rule applies to ionic compounds by determining the structure and stability of the ionic crystal lattice.

The radius ratio rule is a guideline used in the field of crystallography and chemistry to predict the type of lattice structure that an ionic compound will form. It is based on the ratio of the radii of the cation (positively charged ion) and the anion (negatively charged ion) in the compound. This ratio is crucial in determining the coordination number, which is the number of ions of one type that can surround an ion of the opposite type in the crystal lattice.

The rule states that if the radius ratio is less than 0.155, the coordination number is 2, forming a linear structure. If the ratio is between 0.155 and 0.225, the coordination number is 3, forming a trigonal planar structure. If the ratio is between 0.225 and 0.414, the coordination number is 4, forming a tetrahedral structure. If the ratio is between 0.414 and 0.732, the coordination number is 6, forming an octahedral structure. Lastly, if the ratio is greater than 0.732, the coordination number is 8, forming a cubic structure.

The radius ratio rule is important because it helps us understand the stability of ionic compounds. The more closely packed the ions are in the crystal lattice, the stronger the ionic bonds and the more stable the compound. This is because the electrostatic attraction between the ions is maximised, leading to a lower potential energy and greater stability. Therefore, by using the radius ratio rule, we can predict not only the structure but also the relative stability of ionic compounds.