Selenium is a metalloid, which has more nonmetallic than metallic properties. Chemically selenium resembles sulfur, but it is less reactive than this. Nonetheless selenium very rarely occurs in nature in its pure form. Every life form on earth needs selenium in small amounts for different proteins and amino acids. However, if the dose is too high, it quickly becomes poisonous. Hydrogen selenide and many other selenium compounds smell terrible, worse than the accordant sulfur compounds.