why trihalides in group 15 except BiF3 are covalent in nature.

 trihalides in group 15 except BiF3 are covalent in nature.because Bi has large atomic size.in Faugens rule large anionic size and small cationic size is favourable for ionic bond.

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BiF3 is not covalent , ionic because,  according to Fajan's rule, smaller the Cationic size (+ve charge) and greater the anionic size,  the tendency to form covalent bond will increase and vice versa. 

~Bi acquire largest atomic number in nitrogen family so tendency to donate electron increase's  this lead to formation of ionic bond so BiF3 doesn't form covalent bond.

~BiF3 is not covalent because the condition of ionic bond formation according to fajan rule small anionic and large cationic are favourable for ionic bond formation

Bi is the higher number of Nitrogen Group, the tendency of donating electron increases as we move down the group. This leads to the increasing of ionic character in the atom that favours the ionic bond and that is why they are ionic in nature.