When two different halogens react with each other, interhalogen compounds are formed. They can be assigned general compositions as XX’ , XX’_3, XX’₅ and XX’₇ where X is halogen of larger size and X’ of smaller size and X is more electropositive than X’. As the ratio between radii of X and X’ increases, the number of atoms per molecule also increases. Thus, iodine (VII) fluoride should have maximum number of atoms as the ratio of radii between I and F should be maximum. That is why its formula is IF₇ (having maximum number of atoms).

Which is the least basic trihalide of nitrogen?

NF₃

NF₃ is the least basic trihalide of nitrogen because it has different size and the bond is more polar towards nitrogen as it is more electronegative than the remaining halides, which makes electron availability more on nitrogen(in NF₃) than the remaining halides. Thus, it can donate electrons easily and therefore NF₃ is least basic.