3f − is bigger than f because
1. f − has one more electron so there is less effective shielding of the nuclear charge.
2. f − has one more electron which causes greater electron repulsions in the outer orbitals, thus expanding the electron cloud.
3. f − has one less electron so there is more effective shielding of the nuclear charge.
4. f − has one more electron which causes less electron repulsions in the outer orbitals, thus expanding the electron cloud.
5. f − has one less electron so there is less effective shielding of the nuclear charge