THE TRANSITION ELEMENTS

One property of transition metals is the formation of highly colored ions in solution. This photo shows just a few of the colors associated with transition metal ions.

General Introduction to the Transition Elements

The fouth row of the periodic table does not follow the pattern set in the second and third. After the 4s orbital is filled by the elements potassium (K) and calcium (Ca), the next electrons are placed into the 3d orbitals. Because there are five d orbitals in a given shell, the next block of the periodic table is ten elements wide. The ten elements following calcium, as well as the corresponding elements in the fifth and sixth periods, are referred to as the transition metals. The transition elements are some of the most widely used and most common elements on the periodic table, and include iron, copper, silver, and gold. The physical and chemical properties of the transition elements are, in many ways, very different from the physical and chemical properties of the alkali and alkaline earth metals. Some of these differences are discussed in the sections that follow.

The properties of the transition elements do not vary greatly across a period. For example, consider the second period elements. Sodium and magnesium have distinctive metallic properties, silicon is a metalloid, phosphorous and sulfur are solids with nonmetallic properties, and both chlorine and argon are gases. There is a dinstinct progression from elements with metallic properties to elements with nonmetallic properties. In contrast, the properties of the transition metals do not vary greatly moving across a period. In contrast, all of the transition elements have definite metallic properties. Consider again the second period elements. Moving from left to right across the second period elements, there is is a trend towards decreased atomic radius.  Magnesium has a smaller atomic radius than sodium, and aluminum has a smaller atomic radius than magnesium. Moving left to right across the first transition series, however, there is a general trend towards an increase in atomic radius in the first half of the series, amd in the latter half of the series atomic radius remains relatively constant. Other chemical properties, such as ionization energy and electronegativity, remain relatively constant across a transition series.

A major difference between the transition metals and other metals lies with the ability of the transition metals to form coordination compounds. In these types of compounds, molecules with an unshared electron pair, such as water or ammonia, donate the unshared pair to a transition metal or ion. The transition metals are able to accomodate the extra electrons by placing them into vacant d orbitals. The color of transition metals can vary greatly depending upon the coordinated molecules. For example, in dilute aqueous solution copper(II) is pale and nickel(II) is pale green. When ammonia is added, the color of the copper(II) ion changes from pale blue to a much darker blue as the coordinated water molecules are replaced by ammonia. For nickel(II), a change from green to violet as ammonia is added.

Most transition metal compounds are highly colored. In contrast, compounds of the alkali metals and alkaline earth metals are always white. The color of transition metals arises from a split in the energies of the d orbitals caused by coordinated molecules. Oustide of the presence of coordinated molecules, the d orbitals are degenerate, meaning that they have the same energy. In the presence of coordinated molecules, the degeneracy is removed. The energy difference bewteen the orbitals happens to correspond to the energy of visible light.

Lastly, many of the transition metals exhibit multiple oxidation states. The alkali metals and alkaline earth metals exhibit predictable oxidation states; all of the alkali metals form +1 ions and all of the alkaline earth metals form +2 ions. Unfortunately, there is no easy formula for predicting the oxidation states of the transition metals. These range from +1 all the way to +7.


Densities of the First Row Transition Elements
Units are grams per cubic centimeter

Atomic Radii of the First Row Transition Elements
Units are picometers

First Ionization Energies of the First Row Transition Elements

Electron Configurations of the First Row Transition Elements: