Activated carbon can interact with chlorine, bromine and fluorine. Pure bromine is somewhat toxic but less toxic than fluorine and chlorine. Ytterbium forms both dihalides and trihalides with the halogens fluorine, chlorine, bromine, and iodine. Iron's reaction with bromine is less reactive than its reaction with fluorine or chlorine. Almost all known xenon compounds contain the electronegative atoms fluorine or oxygen. Metal fluorides with more fluorine atoms are particularly reactive. All of the lanthanides form trihalides with fluorine, chlorine, bromine and iodine. Addition of fluorine, chlorine, and bromine occurs for C. Such reactions require highly reactive reagents, such as chlorine and fluorine. The second period contains the elements lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon. In addition, fluoride anions are toxic, but not as toxic as pure fluorine. Laboratory studies have also shown that fluorine and iodine atoms participate in analogous catalytic cycles. Many interhalogens consist of one or more atoms of fluorine bonding to a heavier halogen. Its properties are mostly intermediate between those of chlorine and fluorine. Perfluorocarbons (PFCs) are the compounds consisting of fluorine and carbon.