Complex sentence: Caesium is the softest element (it has the hardness of 0.2 Mohs), very ductile, pale metal, which darkens in the presence of trace amounts of oxygen.
Split into simpler sentences: Caesium is the softest element (it has a hardness of 0.2 Mohs). It is very ductile, pale metal, which darkens in the presence of trace amounts of oxygen.