Question

What are Fajan's rules?

Answer 1

An attempt to this question can be made by understanding the use of Fajan's rule. Fajan's rule helps to determine the percentage of covalent character in an ionic compound. An example of Fajan's rule is depicted in comparison of LiCl and NaCl to determine the hardness of compounds.

Complete step by step solution:
We classify compounds as ionic and covalent. However, when we have to determine which compound is the most ionic among few ionic compounds, we use the rules given by Fajan to answer the question.
Fajan's Rules: First Rule :
- It speaks about the polarising power of the cation. If the cation is smaller, then we can say that the volume occupied by the ion is less. If the volume is less, we can conclude that the charge density is relatively high.
- Due to high charge density, we consider that the polarising power of the ion would be high. This increases the covalent character of the compound.
Second Rule:
- The second rule pertains to the polarizability of the anion. If the size of the ion is large, the effective nuclear charge of that particular electron in the valence shell is less. Since the valence electron is loosely bound in large anions, it can easily be polarised by a cation hence increasing the covalent character of the compound.

Note: Fajan gave a third rule which is applied only under certain circumstances. The rule is applied when the anion is the same and the size of two different cations are relatively the same. The rule says that for cations with same charge and size, the cation with electronic configuration ${{(n-1)}{d^n}{n}{s^0}}$which is found in transition elements have greater covalent character than the cation with electronic configuration ${{n}{s^2}{n}{p^6}}$ which is found in alkali or alkali earth metals.