why trihalides except bif3 are predominantly covalent in nature?

Trihalides except BiF3 are predominantly covalent on account of Fajan's rule. Small size of cation, large size of anion, and high charge on both species results in greater covalent character.
 
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BiF3 is predominately ionic because down the grp metallic character increases
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BiF3 is ionic because the size cation (Bi) is quite larger amd size of anion (F) is smaller. Also there is a great difference between their electonegativities. Hence BiF3 is ionic.
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Trihalides are covalent in nature except of BiF3 this is because ,here the cation(Bi) is large in size as compared to anion (F) which is smaller in size .As a result of which no overlapping will take place thus no polarization will occur and therefore covalent character will also not come thus BiF3 will be ionic in nature.
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