Question

Why do covalent characters of lithium halides follow the following order?
LiI > LiBr > LiCl > LiF

Answer 1

Some ionic compounds are not purely ionic in nature, they possess partial covalent character. According to Fajan’s rule, the atomic size of the Lithium is small, therefore it can distort or polarize the electron cloud of anion.

Complete step by step answer: As we know that the compound with cation and anion are considered as the ionic compound and possess ionic character but there are some ionic compounds which carry covalent character. This partial covalent character of the ionic bond is given by Fajan’s rule. The Fajan’s rule says that when the ions of oppositely charged ions are brought together then the bond nature between them depends on the effect of one ion on the other ion.
When the two oppositely charged ions move towards each other, the positively charged ion attracts the electrons of the outermost shell of anion and repels the nucleus carrying the positive charge. This leads to polarization or distortion of anion. If the polarization is small then ionic bond is formed and when polarization is large then covalent bond is formed.
Lithium is a very small ion, therefore it can distort or polarize the anion electron cloud. Therefore, the lithium halide has partial covalent character. The anion with large size can easily get polarized.
As on moving down in the group, the atomic size of the halides increases, therefore the order of atomic size in halides is shown below.
${F^ - } < C{l^ - } < B{r^ - } < {I^ - }$
Fluorine ion is smallest in size and iodine ion is largest.
Therefore, among the given metal halides the covalent character of lithium iodide will be highest and lithium fluoride will have least covalent character.

Note:
The power of the cation to distort the anion is called the polarization power and the ability of the anion to get distorted by the cation is called polarizability. When two ions have the same charge and size, the ion with pseudo noble gas configuration which has 18 electrons in the outermost shell will be more polarizing than the ion with noble gas configuration having 8 electrons.