Posts Tagged ‘Radioactivity’

Science Makes Sense: Week 46:Lanthanides and Actinides

October 31, 2016

I remember seeing the World War II editions of Time magazines among my father’s immense collection of books.  Since they were printed during the war, the sizes were considerably smaller than the regular publication. I think there was an article about plutonium that was used during the war then.

Plutonium is one of the elements found in the Actinide series in the Periodic Table.  Here the 5f orbitals are being filled (from Atomic Number 90-103, i.e.,Thorium, Th to Lawrencium,Lr)  Today we shall also look at the Lanthanide series where the 4f orbitals are being filled from Cerium, Ce (Atomic Number 58) to Lutetium, Lu ( Atomic Number 71) .  The Lanthanide series appear before the transition metal, Hafnium, Hf, in the 6th period, while the Actinide series appear before the transition metal, Rutherfordium, Ru, in the 7th period. These two series are also called the Inner Transition Metals.(Ref.1) In fact the common names are based on the transition metals before these series start: Lanthanum and Actinium respectively.

Lanthanides are formed during the fission of uranium and plutonium.  Lanthanides are like typical metals, silvery -white, they tarnish when exposed to the air, because they form oxides.  They have high melting and boiling points, very reactive and mostly exhibit paramagnetic behavior like oxygen.  Moving from left to right on the Periodic Table, the Lanthanide ions decrease in size, this is called Lanthanide contraction. (Ref.2)

Lanthanides react with water and acids (H+ions) to release hydrogen gas.  They burn easily in air and also react with hydrogen in an exothermic reaction. (Ref.2)

The electro positive nature of the lanthanides and its high reactivity with hydrogen and halides makes it suitable as a catalytic converter in automobiles and in fuel cells.  The ‘f’ orbitals have limited radial extensions; this means that the emission/absorption spectra are sharp, not diffused for the 4f to 4 f levels.  This leads to precise and efficient optical choices of lanthanides in optical instruments ranging from lasers to color television modules. (Ref.3)  Also, lanthanides are used in medicinal applications: as an anti- tumor agent and for kidney dialysis medicine.  Gd, gadalonium, is used in Magnetic Resonance  Imaging  or MRI.  Industrially cerium, Ce,is used in steel manufacturing, since it removes free oxygen and sulfur by forming stable oxysulfides and ties up unwanted antimony and lead. Steel quality is improved when S and O are minimal. (Ref.4)

Actinides are also typical metals, they are soft and silvery- white, have high density and plasticity and can be cut with a knife. Thorium, on the other hand, behaves like soft steel and can be rolled inte sheets or drawn into wires.  Unlike lanthanides, actinides behave like transition metals.  But mostly they are radioactive, paramagnetic and show several crystalline phases.  In fact, actinides have only three naturally occurring elements out of fourteen.  All actinides are radioactive and therefore are toxic and difficult to study (Ref.4).  Just like the lanthanides, actinides react readily with halogens. (Ref.1)  

It is significant to note that the Lanthanides and Actinides are not arranged in columns but rather in two rows.  Much of the chemistry along the row is similar to other members in the row. Elsewhere in the Periodic Table, elements in the same column, not row, have similar properties. (Ref.4)

It is quite amazing that these elements, at the tail end of the Periodic Table, play a controversial as well as a vital role in our lives.

Activities for Middle School Teachers:

Students can practice flash card information on an ‘app’ on smart phones dealing with lanthanides and actinides. (Ref.5)  In addition, students can look at the history of these elements from the time they were discovered and used in constructing bombs for World War II to the use in nuclear reactors and other less controversial applications.

Nuggets of Information:

There are three Lanthanide metals: cerium,Ce,lutetium, Lu and gadolinium, Gd, that have properties similar to the ‘d’ block/ transition metals.  All these three elements contain a ‘d’ electron in their electronic configuration.(Ref.1)

Sometimes these Lanthanide and Actinides are also called rare- earth elements.  However, these elements are not as rare as some of the platinum group noble metals.(Ref.2)

There is an abundance of lanthanides  and the worldwide price is relatively low; substantial ore deposits are found in the U.S.(Ref.3)

Lanthanides exist in the +3 oxidation state since they easily lose two ‘s’ and one  ‘d’ electrons. (Ref.4)

Uranium,U235 is a fissile isotope, which means it can be split through nuclear fission.  Other fissile isotopes are for plutonium Pu, and thorium, Th.  The ability of  an atom to fission depends on the speed of the neutron.  Th requires fast neutrons to fission, while U needs slower neutrons. (Ref.4)

World War II was the first and only time the atomic bomb was used to destroy the enemy.  there were two weapon designs: Assembly” Little Boy” used U 235 and the implosion ” Fat Man” used Pu 239. (Ref.4)