Elements can take many forms, and this tends to dictate the toxicity. For example, potassium metal is extremely reactive, and will explode if it touches water. When you put it in water, it forms potassium hydroxide, which is a caustic base that will burn your skin. If you neutralize the solution with something like hydrochloric acid, then you'll get potassium chloride, which is a harmless salt. You need potassium in your body to live, as it's necessary for your heart to pump and your nerves to fire.