Question

why BiF3 is ionic in nature

Answer 1

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.
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Answer 2

Falling ionization enthalpies as you go down the group. Metallic character increases, and so does ionic character of the trip halides. Electro negativity decreases on moving down a group. In gt 15 Nitrogen at top of gt has the max electro negativity , 3.0 and Bi at bottom, has the lowest electro negativity at 1.8 in group 15. Thus Bi shows ionic compound with Fluorine having electro negativity at 3.98.