Only iodine forms hepta-fluoride IF₇, but chorine and bromine give penta-fluorides. The reason for this is:

Iodine has the largest size among the halogens. Because of its large size, it can easily accommodate 7 fluorine atoms. Both Cl and Br have a smaller size than I, therefore if they try to accommodate 7 fluorine atoms, steric hindrance will take place. Asa result, only iodine forms IF₇.