It has long been presumed that there is a common cause at the genetic, cognitive, and neural levels for autism's characteristic triad of symptoms. However, there is increasing suspicion that autism is instead a complex disorder whose core aspects have distinct causes that often co-occur.
1 Transition metals are kinds of metals that have electrons in d or f orbitals. 2  Transition metals are in columns 3-12 of the periodic table and in the lanthanide and actinide series.  Most transition metals are lustrous, silvery, hard, and good conductors; however, properties may vary greatly.
Metal elements are usually good conductors of both electricity and heat. The dividing line between metals and non-metals is not hard and fast, thus the distinction between Post-transition metals and Metalloids is represented differently on different versions of the Periodic Table. Halogens. 2  The halogen elements are a subset of the nonmetals. 3  They comprise group 17 of the periodic table, from F through At. 4  They generally very chemically reactive and are present in the environment as compounds rather than as pure elements.