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.
Any of the metallic elements within Groups 3 through 12 in the Periodic Table. All the transition metals have two electrons in their outermost shell, and all but zinc, cadmium, and mercury have an incompletely filled inner electron orbital just beneath the outer orbital.