Plutonium
Like other transuranium elements, plutonium can react with water, oxygen, carbon, halogens, nitrogen, and silicon. It forms oxides and hydrides in moist air, causing the plutonium sample to expand by up to 70%. Plutonium from the expanding sample can flake off and catch fire at room temperature. Plutonium is also toxic, although it is less toxic than many people think. The reactivity and toxicity of plutonium make it difficult to handle.