Thallium is a soft and malleable heavy metal with a bluish-white colour. Its relative molecular mass is 204.38. Its most important compound is the colourless, odourless and tasteless thallium(l) sulfate, previously used on a large scale as a rodenticide. Monovalent (thallous) compounds behave like, and may be as reactive as, alkali metals, e.g., potassium, whereas the trivalent (thallic) compounds are less basic, resembling aluminium.