Transition elements, or metals, are a category of materials in the periodic table that are set apart from the "major group" or "A series" elements by their ability to use either the penultimate and outermost electron shells to bond with other substances. In industrial chemistry , transition metals are especially valuable when in their metallic state. Most of the economically important metals (i.e., silver, gold, platinum, nickel, and copper) belong to the transition elements family, as do most of the elements used when high-performance metals are necessary.

One characteristic all the transition elements have in common, besides being metals, is their incomplete d electron orbitals, which fill as atomic number increases. This is partly why transition elements are so useful--the empty orbitals help them accommodate a wide variety of bonding interactions, which makes them good catalysts.

Chemists sometimes differ on which elements should be put in...