Gallium is a Block P, Group 13, Period 4 element. The number of electrons in each of Gallium's shells is 2, 8, 18, 3 and its electronic configuration is [Ar] 3d¹⁰ 4s² 4p¹. In its elemental form gallium's CAS number is 7440-55-3. The gallium atom has a radius of 122.1.pm and it's Van der Waals radius is 187.pm. Gallium is not toxic. Gallium is one of three elements that naturally occur as a liquid at room temperature. The other two are mercury and cesium. The application of gallium that has received the most attention is the production of semiconducting compounds. Of these, the most important are the compounds of gallium with antimony, arsenic or phosphor. Nowadays gallium arsenide (Ga-As) is undoubtedly the most used. This compound is used in the production of several electronic parts such as diodes and transistors, made for voltage rectification, signal amplification, etc. Other gallium arsenide applications are the semiconductor "lasing" and microwave generation and also in sensors to measure temperature, light or magnetic field. Gallium was first discovered by Hans Christian Oersted in 1825. See Gallium research below.