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CHEMISTRY AND PHYSICS

Searching for the magic numbers

Produced artificially, four chemical elements are now part of the periodic table; the search for other, more stable elements continues

Mendeleev in 1897: almost three decades after organizing the elements based on their physical and chemical characteristics

WikipediaMendeleev in 1897: almost three decades after organizing the elements based on their physical and chemical characteristicsWikipedia

2016 began with newspapers announcing the redecoration of chemistry laboratory walls all over the world.  This is because the posters displaying the famous periodic table — the diagram that classifies the known chemical elements by their characteristics and properties — were suddenly out of date.  In a press release on December 30, 2015, the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP) officially recognized the existence of four chemical elements discovered in recent years.  They are the elements numbered 113, 115, 117 and 118, still awaiting official names, now added to the 114 elements identified previously.

The new chemical elements are called super-heavy because their nuclei contain an elevated number of protons (particles with a positive electric charge), much higher than that of chemical elements found in nature.  It is this number of protons, known as the atomic number, that distinguishes one chemical element from another and defines many of its characteristics.  For example, carbon, which accounts for most of the mass of living beings, has only six protons in its nucleus.  At room temperature and in a pure state, carbon forms crystals that can be soft and black, such as in graphite, or transparent and hard, such as in diamonds, depending on how the atoms are organized geometrically.  At the other end of the spectrum, the heaviest natural chemical element, uranium, is a very dense, radioactive metallic solid.  It has 92 protons and, thus, is much lighter than the four just added to the periodic table.

The new elements are very difficult to observe and probably do not exist spontaneously in nature — at least not for very long.  Since their nuclei are super-heavy, they are so unstable and fleeting that they break apart in fractions of a second.  Their existence was only confirmed through a series of experiments performed over the last decade.

One of the few laboratories able to manufacture these elements is in the Riken Institute, in Japan.  It was there that element 113 was identified in 2004.  Other laboratories with the same capabilities are in the Nuclear Research Institute, in Dubna, Russia, and in centers in the United States.  A collaboration between a team in Dubna and U.S. researchers, most at the Lawrence Livermore National Laboratory, produced element 115 in 2004, element 118 in 2006, and element 117 in 2010.

With the four new chemical elements, added to elements 114 and 116, whose existence was recognized in 2011, all of the empty squares in the seventh line of the periodic table have finally been filled.  “Just in the last 50 years, 17 new chemical elements have been added to the table, increasing the total from 102 to 118,” says physicist Edilson Crema, of the University of São Paulo (USP) Physics Institute.

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“When French chemist Antoine Lavoisier published his Elementary Treatise of Chemistry in 1789, considered a landmark in modern chemistry, the work listed only 33 elements,” observes chemist and science historian Carlos Alberto Filgueiras, of the Federal University of Minas Gerais (UFMG).  During that era, the identification of new chemical elements depended on the development of extraction products and methods to study minerals.  “The analysis of the properties of new minerals often revealed the presence of a previously unknown element,” he explains.

The periodic table only appeared in the late 1860s.  Chemists had already noted that the elements, ordered by increasing atomic mass (the sum of their protons and neutrons), formed series with similar physical and chemical properties that repeated periodically as the number increased.  Based on these observations, Russian chemist Dmitri Mendeleev ordered the 65 elements identified up until then in what he called the periodic table of chemical elements.  He predicted the existence of others — such as gallium and germanium — that were discovered only years later.

After filling in almost all of the gaps in the periodic table between hydrogen, which has one proton, and uranium, with 92, researchers began using particle accelerators in the 1940s to try to produce chemical elements heavier than uranium.  The first synthetic chemical elements were formed by adding a neutron which, when attaching to the nucleus, converts into a proton, releasing an electron and a neutrino.  This strategy worked until fermium, which has 100 protons.  From then on, heavy elements were created by the collision and fusion of two lighter nuclei.

Production of these elements requires a fine adjustment of the masses of the nuclei and the energy with which they are shot at each other.  This is because the collision must be energetic enough to overcome the repulsive force between the nuclei, which have a positive electric charge.  But the energy cannot be so high as to prevent the formation of a larger, stable nucleus, though if only for a moment.  The objective of the physicists is not just to manufacture new chemical elements.  This is also a way to test the theories on how protons and neutrons interact and how the material behaves on an even more elementary level.  These theories explain how the lighter elements, such as hydrogen, helium and lithium, formed during the Big Bang — the explosion believed to have occurred when the Universe was created — and later produced the other elements through nuclear fusion inside stars and during the explosions that extinguish them.

The nucleus of an atom is under constant tension.  The protons repel each other because they have the same electric charge, positive.  They only remain united by the action of an opposing attractive force, called the strong nuclear force.  This equilibrium between the forces is very delicate.  According to Crema, nuclei contain a number of neutrons — electrically neutral particles — in addition to protons.  “Neutrons are a kind of nuclear stabilizer,” he says.  “Nuclei with a lot of protons require an even larger number of neutrons in relation to the number of protons, which makes it harder to form super-heavy nuclei.”

A theory called the shell model proposes that, in atomic nuclei, the protons and neutrons are organized into concentric shells, each one containing a maximum number of particles, called the magic number.  According to this model, the more complete the external shell of a nucleus, the more stable it is.  This idea, in principle, explains why some heavy nuclei break apart easily, while others last longer.  Physicists hope to manufacture elements containing magic numbers of particles.  They might be able to remain stable for several years, and would allow the establishment of an eighth or even ninth line in the periodic table.  “But this,” says Crema, “is still just conjecture and hope.”

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