Alex Behm
Scott Glaser
Dan Tobert
Kevin Vanden Bosch
Lanthanides and Actinides
The Lanthanides and Actinides are part of a unique section of the periodic table.
These compounds have partially filled f orbital. This makes them analogous to the
transition metals, which have a partially filled d orbital. The Lanthanides are also known
as the “rare-earth” metals, however this nickname is not very accurate as many of the
Lanthanides are abundant in the earth’s crust, with some present in even greater
quantities than lead. The Lanthanides and Actinides differ from the other elements in
several key ways. First, they are not arranged in columns but rather in two groups. Much
of the chemistry of individual elements across the row is very similar to other members in
that row, similar to elements in the same column elsewhere in the periodic table. For
instance, almost all of the Lanthanides exist in the +3 oxidation state due to the fact that
they easily lose their two s electrons and a d electron. Another trait that is often studied
in laboratory classes is the Lanthanide contraction. This is the observation that the ionic
radius of all the Lanthanides decrease as you go from left to right across the row. This is
due to the increasing Zeff. Actinides, on the other hand, have only three naturally
occurring elements out of 14. All of the elements past Uranium (z=92) do not occur
naturally. All of the actinides are radioactive and therefore are toxic and difficult to
study. Some of the industrial and medical uses of Lanthanides and Actinides are
discussed in this paper.
The 235Uranium is a fissile isotope of the element Uranium. This isotope can be
split through nuclear fission, which is the process of splitting atoms or fissioning them.
Other fissile isotopes are 239Pu and 232Th. The ability for an atom to fission depends upon
the speed at which the neutron is moving.
232
Th requires a very fast neutron to induce
fission but 235U needs slower neutrons. If a neutron is too fast, it will pass right through a
235
U atom without affecting it at all. The fissioning of 236U can produce over 20 different
products that always add up to an atomic mass of 236.
World War II sparked the first and only time that the Atomic Bomb had been used
in war to destroy the enemy. Two weapon designs were available in WWII; the gun
assembly “Little Boy” A-Bomb that used 235U and the implosion assembly “Fat Man”
that used 239Pu. “Little Boy” was dropped on Hiroshima, Japan on August 6, 1945 and
three days later “Fat Man” was dropped on Nagasaki, Japan. These bombings ended
WWII.
In a nuclear bomb, each nuclei undergoes fission releasing 2 to 3 neutrons which
then all stimulate additional fission reactions that are uncontrolled. These reactions
multiply exponentially and within a few milliseconds, billions of nuclei fission
simultaneously, releasing a tremendous amount of energy and heat resulting in a nuclear
explosion. In the gun assembly one subcritical mass (sample in such a configuration that
the multiplication factor is less than 1) of 235U is driven by an explosive charge into
another to create a supercritical (greater than 1) mass. In the implosion assembly using
239
Pu, a bomb can only be created by surrounding a slightly subcritical sphere with
explosives. When those explosives are detonated, the shock wave crushes the Pu sphere
to a high enough density that it becomes supercritical and the inward motion of the Pu
prevents it from flying apart before it completely reacts. Although these bombs seem
easy to make, fissile 235U is almost identical to its nonfissile isotope 238U and the 239Pu
bomb design is very difficult because to create an explosion that will symmetrically
compress a sphere is almost impossible.
In an industrial application of the lanthanide group, Cerium is used in steel
manufacturing. Cerium is the most abundant rare earth element. Cerium is referred to as
cerous in the +3 oxidation state and ceric, the metal, in the +4 oxidation state. When
Cerium is added to molten steel it combines with and removes free oxygen and sulfur by
forming stable oxysulfides and by tying up some undesirable trace elements, such as lead
and antimony. Steel quality is improved when the oxygen and sulfur content is minimal.
Lanthanides are used in many medicinal applications, such as in antitumor agents, and kidney dialysis medicine. One of the most known application of these
elements is the use of Gadolinium in Magnetic Resonance Imaging (MRI). Gadolinium in
an oxidation state of 3+ (Gd3+) is used as a contrasting agent during an MRI.
MRIs work using magnets and a radio frequency pulse, much in the same way an
NMR works. They are used to distinguish normal masses from abnormal masses within
the body to locate problem areas.
First, a magnet is used to align the Hydrogen protons from water either with or
against the magnet. The majority of these protons have an overall effect of canceling each
other out to a neutral alignment. Then a radio frequency pulse is let out and the protons
begin to spin. When the pulse is shut off, the protons gradually revert back to their natural
alignment within the magnetic field. When they convert back to their natural alignment,
they let off a characteristic energy which is recorded and mapped on an image.
Gadolinium is injected intravenously before the image is taken. The injected
Gadolinium travels and accumulates in abnormal masses within the body. Because of the
paramagnetic properties of Gd3+(It has 7 unpaired f-electrons!), the element works
amazingly well as a contrasting agent. The electron spins of the paramagnetic center of
the contrast agent and the proton nuclei of the water protons interact to cause the
relaxation rates of the nearby water protons to increase. The increased relaxation rates of
water cause by the paramagnetic properties of Gadolinium cause the abnormal masses in
an MRI image to appear brighter than those not containing the Gadolinium contrasting
agent (normal masses).
Lanthanides are often used for there fluorescent properties. The Lanthanides
fluoresce easily because of the f to f transitions that occur. A myriad of ligands can be
attached to the central metal. These ligands are often designed to attach to a certain
molecule or type of molecule and then can be induced to fluoresce, thus serving as a
marker for a specific type of molecule. Europium compounds, for example are often
used in molecular genetics to mark specific strands of DNA. Europium oxide was also
used in cathode ray television sets as the red glowing dye in the trichromatic setup.
In conclusion, Lanthanides and Actinides are unique in the fact that they are
relatively rare compared to other elements. In addition, the f orbitals that these elements
possess give rise to uniformity in oxidation states for the Lanthanides. The degree of
radioactivity has been useful in some cases (nuclear energy for example) but also harmful
because it has impeded the degree to which these elements can be studied.
Works Cited:
Shriver and Atkins Inorganic Chemistry
http://nuclearweaponarchive.org/Usa/Med/Lbfm.html
http://www.visionlearning.com/library/flash_viewer.php?oid=3602&mid=59
Six Ideas That Shaped Physics Unit Q: Particles Behave Like Waves
www.wikipedia.com
http://www.coe.waseda.ac.jp/matsumoto/1.html
http://www.science.co.il/PTelements.asp
http://jbfpc2.ycp.edu/gchem/gdpaper.pdf
Orvig, C., Thompson K.H. (2006)Lanthanide