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.
Transition Metals vs. Main-Group Elements. There is some controversy about the classification of the elements on the boundary between the main group and transition-metal elements on the right side of the table. The elements in question are zinc (Zn), cadmium (Cd), and mercury (Hg).