Complex sentence: Bismuth is an exception: bismuth pentafluoride is not as acidic as antimony pentafluoride because it is polymeric bismuth's trifluoride is ionic (the element is a borderline metal).
Split into simpler sentences: Bismuth is an exception: bismuth pentafluoride is not as acidic as antimony pentafluoride because it is polymeric. Bismuth's trifluoride is ionic (the element is a borderline metal).