Final answer:
Solid iodine is brittle due to weak dispersion forces between iodine molecules in its molecular lattice structure, which when stressed can cause layers to shift and fracture.
Step-by-step explanation:
Iodine is brittle in its solid state because of its molecular lattice structure, where individual iodine molecules are held together by relatively weak dispersion forces. Despite being the strongest within the halogen group, these forces are not strong enough to allow for flexibility, so when iodine crystals are subjected to mechanical stress, layers of molecules can shift, leading to fracture and resulting in the brittleness observed. Iodine's electron configuration, with -empty valanced orbitals, allows for these molecular interactions to occur in its solid state.