Number of electrons in N3- = 7+3 = 10Number of electrons in O2- = 8+2 = 10Number of electrons in F- = 9+1 = 10Since, all the three species have each 10 electrons hence they are isoelectronic species.It is considered that, in case of isoelectronic species as the negative charge increase, ionic radii increase and therefore the value of ionic radii areN3- = 1.71 (highest among the three)O2- = 1.40F- = 1.36 (lowest among the three)N3- > O2-> F-