The so-called transition elements, or transition metals, include the first three members of groups 4 through 11, the elements from titanium to copper, and those positioned in the radials adjacent to each. Known as the elements of the first, second, and third transition series, these metals are characterized by the filling of an inner d electron orbital as atomic number increases. Many of the chemical and physical properties of the transition elements are due to their unfilled d orbitals. In the elements of the lanthanon series and the actinon series the inner f orbital is filled as atomic number increases; those elements are often called the inner transition elements. Members of the transition series generally exhibit high density, high melting point, magnetic properties, variable valence, and the formation of stable coordination complexes. Their variable valence is due to the electrons in the d orbitals. Many of the complex ions and compounds formed by transition metals are highly colored and exhibit paramagnetism. Click on any of the headings in the box above for more detail.