atomic number 9 - a nonmetallic univalent element belonging to the halogens; usually a yellow irritating toxic flammable gas; a powerful oxidizing agent; recovered from fluorite or cryolite or fluorapatitechemical element, element - any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter cryolite, Greenland spar - a white mineral consisting of fluorides of aluminum and sodium; a source of fluorine fluorapatite - a form of apatite in which fluorine predominates over chlorine fluor, fluorite, fluorspar - a soft mineral (calcium fluoride) that is fluorescent in ultraviolet light; chief source of fluorine gas - a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely halogen - any of five related nonmetallic elements (fluorine or chlorine or bromine or iodine or astatine) that are all monovalent and readily form negative ions |