to the Periodic Table

to the 15 th and 17 th groups of the periodic table respectively and are expected to share some of their chemical and physical properties with the oth...

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FEATURE ARTICLE M.S.S. MURTHY

New Members to the

Periodic Table F

OUR new elements discovered in December 2015 have recently been named by the International Union of Pure and Applied Chemistry (IUPAC). The names are: Nihonium (113Nh), Moscovium (115Mo), Tennessine (117Te) and Oganesson (118Og). They complete the 7th period of the periodic table and constitute the heaviest elements known today. With the discovery of these four new chemical elements, the total number of elements in the periodic table has gone up to 118. The credit for synthesising the four new elements goes to scientists from Japan, USA and Russia. During the years 2000 to 2012, Japanese scientists at RIKEN nuclear research laboratory bombarded a thin target of bismuth ions (83 protons and 126 neutrons) with high-speed zinc ions (30 protons and 40 neutrons) accelerated to 30,000 sq km to create a new element of atomic number 113. The element was later named ‘Nihonium’ (Nh), in honour of their country. Nh-278 is highly radioactive with a half-life of only 0.24 millisecond and decays through a series of alpha particle emissions to ‘Mendelevium’ (Md - proton

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number 101 and neutron number153). During the course of their experiments the Japanese scientists identified six isotopes of Nh, all radioactive. The most stable was Nh-286 with a half life of 20 milliseconds. With its atomic number 113, Nh is placed in the 13th group of the periodic table along with elements like boron, aluminium, gallium, etc. Hence, it is expected to be a metal, solid at room temperature. Its other properties like boiling point, melting point, oxidation status, etc. are expected to follow periodic trends. Syntheses of elements 115 and 117 were carried out in collaborative research between the Joint Institute for Nuclear Research (Dubna, Russia), and Lawrence Livermore National Laboratory, Tennessee, USA in 2003 and 2010 respectively. The researchers bombarded targets of Americium-243 and Berkelium-249 with high-speed Calcium-48 ions in a cyclotron. The elements were named ‘Moscovium’ (Mc- atomic number 115) and ‘Tennessine’ (Ts- atomic number 117) respectively in honour of the cities Moscow in Russia and Tennessee in USA where the participating laboratories are situated. They were able to identify four isotopes of Moscovium, Mc-287 to Mc-290, all of which decay by alpha emission to various isotopes of

Nihonium. Mc-290 had the longest half-life of 0.8 seconds. Element 117, Tennessine was found to have two isotopes Ts-293 and Ts-294 which decay to Mc-289 and Mc-290 with half-life 20 and 50 milliseconds respectively. Moscovium and Tennessine belong to the 15th and 17th groups of the periodic table respectively and are expected to share some of their chemical and physical properties with the other elements in the groups. The last of the four new elements, element-118 was first synthesised in 2002 by a team of Russian and American scientists at the Joint Institute of Nuclear Research in Dubna, Russia. It was named ‘Oganesson’ (Og-118) in honour of the Russian physicist Yuri Oganessian, leader of the team. It was synthesised by projecting high speed calcium ions on to a target of californium. It has the highest atomic number and highest mass number of all known elements. Being highly unstable it decays with an estimated halflife of 0.7ms to livermorium (atomic no. 116 and mass no. 290 – another super heavy element). Og is a member of the 18th group, the noble gases, but is significantly more reactive than the other members of the group. Further, it is thought to be not a gas, but solid at room temperature. Earlier in 2011 the IUPAC had

The last of the four new elements, element-118 was first synthesised in 2002 by a team of Russian and American scientists at the Joint Institute of Nuclear Research in Dubna, Russia.

recognised the discovery of elements flerovium (Fl) and livermorium with proton numbers 114 and 116 respectively; these are members of the 7th period of the periodic table. With the discovery of the current four new elements, also of the 7th period, the 7th period is complete. Now there are no blanks in the periodic table. What next? Is there an 8th period? How big does the periodic table get? These are some of the interesting questions that researchers are trying to answer. A study of the periodic table shows that the stability of the nucleus decreases sharply with the increasing atomic number beyond the heaviest naturally occurring element uranium (atomic number 92). For example, isotopes of elements with atomic number greater than 101 decay with half-lives less than one day, some with only a few milliseconds (with the exception of dubnium – atomic number 105 and mass number 268 – which has a half-life of about 30 hours). The latest four elements confirm this trend. As more and more protons and neutrons are packed into the nucleus to create heavier and heavier nuclei, the short range nuclear force which holds these particles together weakens towards the edge of the nucleus, rendering it highly unstable. The nucleus

moves towards stability through rapid radioactive decay. Nevertheless, researchers suspect that there can be a group of super heavy elements that defy this trend to exhibit greater stability and thus longer halflives. In the electron configuration of atoms the octet rule states that atoms become stable when the valence shells gain a full complement of valence electrons, for example the noble gases. Similarly, it is suggested that atomic nucleus is made up of individual shells that have different energy levels. Certain specific numbers of protons and neutrons in these energy levels may render the nucleus particularly stable, creating what is known as an “Island of stability”, where the nucleus may exhibit longer half-life than expected. Glenn Seaborg, who pioneered the synthesis of transuranium elements, suggested that such ‘magic numbers’ could be 184 for neutrons and 114, 120, and 126 for protons. One such element is flerovium with proton number 114. It is predicted to be near the edge of the island of stability. Six isotopes of flerovium have been identified so far, ranging in mass number from 284 to 289 and with increasing half-lives from 2 ms to 2 seconds, the heaviest being the longest lived. It is expected that heavier flerovium isotopes, especially

the doubly magic flerovium-298, which is yet to be synthesised, may have even longer half-life. The other members of the island of stability would be elements with proton number 120 and neutron number 184; proton number 126 and neutron number 184. These elements also have not yet been synthesised. The Russian scientist Yuri Oganessian, one of the cosynthesisers of flerovium and after whom the element 118 is named, suggests that there can be a second island around proton number 164. Paul Karol, Chair of the IUPAC says that even though some attempts have been made to synthesise elements with proton numbers 119 and 120, synthesising the elements on the island of stability may prove to be very difficult. This is because the nuclei available as starting material do not deliver the necessary sum of neutrons. It is virtually impossible with the current methods to produce such nuclei. He feels newer technologies are required. Since these super heavy elements are produced so few in numbers at prohibitively high cost and decay so fast, one may ask what is the their use? It is true they have no practical use. But researchers think that a study of the decay of these massive elements may give further insight into the forces that hold the subatomic particles together in the nucleus, the structure of the nucleus and the processes involved in nucleosynthesis. These are fundamental questions in nuclear physics. And so the effort will go on. M.S.S. Murthy, B-104, Terrace Garden Apartments, 2nd Main Road, BSK IIIrd Stage, Bangalore-85

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