Archive for October, 2016

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)




Science Makes Sense-Week 45: Oxygen the giver of life

October 17, 2016

When school was over on those hot days in summer, we would all cram into the buses that had limited seats and stand holding on to the metal poles or leather grasps.  Invariably, one of us would sway and faint.  I learned later that it was due to lack of oxygen to the brain and that can occur in a crowded space.

Joseph Priestley discovered this element in 1774 and answered a question that had been vexing scientists and philosophers for a long long time:”How and why do things burn?” Yet, even though he invented many things and was a pioneer in changing the idea of what the composition of air was, he still believed in the phlogiston theory and called oxygen ‘dephlogisticated air’. (Ref.1)

The phlogiston theory is a theory prevalent in 19th century chemistry.  Every combustible substance was supposed to contain phlogiston and this would be removed during combustion.  (Ref.2)   The erroneous idea prevalent because of this theory was that substances lost weight after combustion since usually the ash or residue left behind is lighter than the original substance. But, in reality, there is a net weight gain, because of the formation of oxides, that is not observed since some of the by-products of combustion could be gases that escape.  Hence oxidation was considered to be the removal of phlogiston and reduction was combination with phlogiston! (Ref.2)

Oxygen,O2 ,is a colorless, odorless gas,but the liquid and solid form is pale blue in color. (Ref.3) The electronic structure of oxygen is 1s2 2s2 2p4 which indicates that there is an unpaired electron making oxygen attracted to a magnetic field.  This is explained later.(Ref.4)

All living beings need oxygen to breathe.  Oxygen supports combustion, combines with most elements and is a component of hundreds of thousands of organic compounds.  This gas at room temperature was the atomic weight standard  for all the other elements till 1961.  Then IUPAC (International Union of Pure and Applied Chemistry) adopted carbon 12 as the new standard.(Ref.3)

Oxygen enrichment of steel blast furnaces is a major use of this gas.  Large quantities of  oxygen are needed in the synthesis of ammonia,methanol and ethylene oxide.  It is also used as a bleach, for oxidizing oils, oxy-acetylene welding and determining the carbon content of steel and organic compounds.(Ref.3)

When you look at the electronic configuration of the oxygen molecule, O2, it has two unpaired electrons in the ‘p’ orbital. Meanwhile, for the nitrogen molecule, N2, all the electrons are paired (three unpaired in the atom, that becomes six in the molecule  hence all paired in the ‘p’ level.)  The unpaired electrons in the oxygen molecule move around in their orbitals; this orbital motion generates a loop of current which leads to a magnetic field.   Now the electrons are arranged randomly , causing a cancellation of magnetism.  However, when an external magnetic field is applied, these tiny electrons align themselves in the same direction and the oxygen molecules are attracted to the external magnetic field.  This type of magnetism is called paramagnetism.( Ref.4)

In the case of nitrogen, since all the electrons are paired, half the electronic spin is clockwise and the other half is counter clockwise.   When an external magnetic field is applied, the induced magnetic field is in the opposite direction and repel the applied magnetic field, this is called diamagnetism.(Ref.4)

We already know that oxygen is vital for human respiration. Oxygen therapy is used for people who have trouble breathing due to say, emphysema or pneumonia. Pure oxygen is used also in space suits so the astronauts can breathe. (Ref.5). More uses of oxygen are mentioned in the Nuggets.

While we talk about how this element is used, we must not forget that the everyday action of photosynthesis releases oxygen.   Photosynthesis is the process by which green plants use the energy in sunlight to synthesize material from air and water for growth.   Violet light is absorbed by chlorophyll and the energy obtained is used to split water  when oxygen is given off by the reaction.   Sunlight causes green leaves to produce oxygen.(Ref.6)

We cannot imagine life on earth without this invaluable element.

Activities for Middle School Teachers:

Students need to do Truth Tables to understand math logic  and reasoning.   How would one understand erroneous theories like the phlogiston theory or the geocentric theory using these tables?   What causes certain erroneous theories to survive for a long period?  Is science truly objective sometimes?

Students must be already aware of magnetic substances like iron,nickel and cobalt.  These elements exhibit ferromagnetic behavior. Study the different forms of magnetic behavior.(Ref.7)

Nuggets of Information:

Joseph Priestley,though an Englishman by birth, had to move to America since he gave vocal support to the French and American revolutions. He invented carbonated water and the rubber eraser.(Ref.1)

Lavoisier,the French chemist named the gas oxygen.  He did not believe in the phlogiston theory and burned all books associated with it.(Ref.1)

Regular oxygen is diatomic, i.e., every molecule of oxygen has two atoms in a covalent bond.  Ozone is triatomic,O3,  which is highly reactive, formed by the action of an electric discharge or ultraviolet light on oxygen.  The name is derived from the Greek phrase meaning’I smell’.( Ref.3)

Two-thirds of the human body and nine- tenths of the mass of water is made up of oxygen.(Ref.3)

Oxygen tanks  are present in submarines and airplanes for emergency use.(Ref.5)

Rockets use liquid oxygen to burn fuel and generate lift.(Ref.5)

Scientists use two isotopes of oxygen (16,18) in skeletons to determine the climate thousands of years ago!(Ref.5)

Oxygen gas is poisonous to bacteria that causes gangrene; therefore, it is used to kill gangrene.(Ref.5)

References: is chemistry theory under”Why is O2 paramagnetic while N2 diamagnetic”) ( type in ‘ uses of oxygen’)