Chemical or element? If element, any of the trans-uranium elements are highly dangerous as they give out such intensely dangerous radiation. An amount of americium, curium or neptunium that you cannot even see would kill you within minutes – and you wouldn’t even have to touch or swallow it, simply stand within a few metres of it without radiation protection. As for chemicals? I would vote for botulinum toxin, a protein produced by the bacterium Clostridium botulinum. An ounce carefully (!!) administered would kill everybody on the planet. The bacterium can found in tinned food that hasn’t been prepared properly and causes “botulism” a potentially fatal illness. Fortunately the bacterium is killed by heating to 80C and by oxygen (we call it an “obligate anaerobe”). “Botox” is an extact of the bacterium and removes lines on the face by paralysing the muscles below the skin. The things some people will do to try to look young!!!!