Metallic chemical element, chemical symbol In, atomic number 49. Of a brilliant, silvery-white lustre, it is so soft that it can be scratched with a fingernail. Its most common isotope, indium-115, is very weakly radioactive, with a half-life measured in billions of years. Like tin, the pure metal emits a high-pitched cry when bent, and, like gallium, molten indium wets glass and other surfaces, which makes it valuable for producing seals between glass, metals, quartz, ceramics, and marble. The metal is used in coating high-performance engine bearings and is an ingredient in low-melting-point alloys for sprinkler heads, fire-door links, and fusible plugs. In various combinations with elements such as gallium, phosphorus, and arsenic, it forms compounds having semiconductor properties useful in electronics, including solid-state light-emitting devices. Transparent electrodes made from an oxide of indium and tin are widely employed in liquid crystal displays.