A chemical compound in which one atom is enclosed within a larger cluster of atoms that surrounds it like an envelope. The term comes from the Greek word chela, meaning claw. Chelating agents—compounds that can form chelates with other atoms—have a wide variety of environmental applications. For example, the compound ethylenediaminetetraacetic acid (EDTA) is used to remove lead from the blood. EDTA molecules surround and bind to lead atoms, and the chelate is then excreted in the urine. EDTA can also be used to soften hard water by chelating the calcium and magnesium ions that cause hardness.