transition elements or transition metals, in chemistry, group of elements characterized by the filling of an inner d electron orbital as atomic number increases. This includes the elements from titanium to copper, and those lying in the columns below them in the periodic table. Many of the chemical and physical properties of the transition elements are due to their unfilled d orbitals. In the elements of the lanthanide series and the actinide series the inner f orbital is filled as atomic number increases; those elements are often called the inner transition elements. Transition elements 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. The study of the complex ions and compounds formed by transition metals is an important branch of chemistry. Many of these complexes are highly colored and exhibit paramagnetism.