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The Race for New Chemical Elements

The Race for New Chemical Elements

COVER STORY

Periodic Table of the Elements

Dmitri Ivanovich Mendeleev (https://www.sciencephoto.com)

To celebrate the 150th anniversary of the of elements, The Race for the United Nations has proclaimed 2019 as the International Year of the Periodic Table. However, New Chemical scientists claim that the Periodic Table is far from being complete. And an interesting race is on Elements worldwide to synthesise new elements. Ramesh Chandra Parida

14 | Reporter | May 2019 YSTEMATISATION is an essential part of the scientific Table 1. List of man-made elements, their symbols and the knowledge that makes the study of science easier and year of discovery Sguides it into the future. Many eminent scientists have made their epoch-making contributions to achieve it in various Atomic Year of fields of science. Undoubtedly one of the foremost among number Name Symbol discovery them is the Russian Chemist Dmitri Ivanovich Mendeleev, who designed the “Periodic Table of Elements” a century and 93 Np 1940 half ago (on 17 February 1869) making the study of chemistry 94 Pu 1940-41 systematic. 95 Am 1944-45 In fact, the process began with Dobeveiner. He classified certain elements with similar properties into groups of three, 96 Cm 1944 called triads. Then Newland (1863) observed that if the elements 97 Bk 1949 are arranged in the order of their atomic weights, the 8th element starting from a given one is a kind of repetition of the first, 98 Cf 1950 like the 8th note of music and he called it the Law of Octaves. 99 Einsteinium Es 1952 Then Lothar Meyer (1869) plotted a graph of atomic 100 Fm 1953 volumes versus atomic weights of different elements and found that similar elements occupied similar positions on the curve. 101 Md 1955 The same year Mendeleev proposed his famous periodic law, 102 No 1958 which stated: “Properties of elements are a periodic function of their atomic weights; similar elements are repeated at regular 103 Lr 1961 periods or intervals.” 104 Rf 1965 He arranged the 63 elements, known at that time, in the 105 Db 1967 ascending order of their atomic weights in a table consisting of 9 vertical columns, called groups and 7 horizontal rows 106 Sg 1972 called periods. In a , elements exhibited gradual variation 107 Bh 1981 in their properties or in other words, periodicity of their 108 Hs 1982 behaviours. In order to maintain such conditions, Mendeleev had to leave certain places vacant in it, which he predicted 109 Mt 1984 would be filled by new elements to be discovered later. The 110 Ds 1994 table was called “Mendeleev’s Periodic Table”. 111 Rg 1994 Modern Periodic Table 112 Cn 1996 During the days of Mendeleev the that indicates 113 Nh 2004 the number of (or ) present in an element and their arrangement in different orbitals, which actually 114 Flerorvium Fl 1998 determines its chemical and physical properties, was unknown. 115 Moscorium Mc 2004 Therefore, in 1913 Henry Moseley changed the basis of classification of the elements replacing their atomic weights by 116 Lv 1998 atomic numbers, which formed the “Modern Periodic Table”. 117 Ts 2010 It retained many of the characteristics of its predecessor 118 Og 1998 and also had vacant positions, which were gradually filled up with newly discovered elements like , Scandium, Germanium, etc. And thus the process continued. And so, at So far essentially all the elements after (U92) present the Modern Periodic Table has been expanded to 18 have been synthesised at four laboratories: the Lawrence groups and accommodates 118 elements of which the first 92 Berkeley National Laboratory in the US (elements 93 to 101 from (H1) to Uranium (U92) occur in and the and jointly 103 to 105); the Joint Institute for Nuclear Research rest have been synthesised. in Russia (elements 102, 114 to 118 and jointly 103 to 105); The first man-made element Neptunium (Np93) was the GSI Helmholtz Centre for Heavy Research in Germany synthesised in 1940 and then gradually the list elongated to (elements 107 to 112) and the in (element 113). Oganesson (Og118), which has led to the completion of the 7th All these have been named by the International Union of Pure period of the periodic table (Table 1). and Applied Chemists (IUPAC). Scientists have, however, not stopped there yet. They are in the process of synthesising elements with atomic numbers Leap into Future from 119 to 122 and proposing to go beyond those, so that an After the synthesis of Og118 the 7th period of the periodic table additional new period (8th) can be added to it.

May 2019 | Science Reporter | 15 Some laboratories that synthesised chemical elements (From top left, clockwise): Lawrence Berkeley National Laboratory, US; Joint Institute for Nuclear Research, Russia; GSI Helmholtz Centre for Heavy Ion Research, Germany and RIKEN in Japan was completed. However, at present attempts are underway to properly with the currently available technology, decaying synthesise the next higher elements with atomic numbers 119 within a microsecond, before reaching the detectors. Therefore to 124. By now scientists have come very close to achieving the possibility of detection and study of the elements with their objectives and have already predicted their properties atomic numbers from 121 to 124 will depend greatly on the (Table 2). This has begun the addition of a new period, the improvement of the technology and the theoretical model 8th, in the Periodic Table and has opened up the challenge to being used. synthesise the next 14 elements to complete it. The super heavy elements are synthesised by nuclear Element 119 fusion – “hot” and “cold”. In hot fusion reactions, very light, The first attempt to synthesise the element 119 or Ununennium high energy projectiles are accelerated towards very heavy (Uue119) was made in 1985 at the super HILAC accelerator, targets (like the ) giving rise to compound nuclei at Berkeley, , by bombarding a target of Einsteinium higher excitation energy (40 to 50 MeV) that may undergo with 254 (Es254) with 48 (Ca48) , fission or alternatively, evaporate several (3 to 5) . but no atom was detected. Then several experiments were On the other hand, reactions use heavy conducted in Russia until 2011, without much success. projectiles, usually from the 4th period and lighter targets like A year after, in 2012 attempts to synthesis its with

Lead or . The fused nuclei thus produced have relatively mass numbers 295 (Uue295) and 296 (Uue296) was made at the The first attempt to synthesise element 119 or low excitation energy (10 to 20 MeV), which decreases the GSI Helmholtz Centre for Heavy Ion Research in , Ununennium (Uue119) was made in 1985 at the super HILAC accelerator, Berkeley, California probability that these products will undergo fission reactions. Germany by bombarding a target of Berkelium-249 (Bk249) with

Therefore, hot fusion reactions tend to produce more - -50 (Ti50). Finally, in December 2017, the RIKEN rich products, because the actinides have the highest neutron-to- Laboratories in Japan began the renewed search for element 119 ratios of any elements that can be made in microscopic by bombarding Curium-248 (Cm248) with Vanadium-51 (V51) quantities. beam and based on their results, it was predicted that both the Elements with atomic numbers 119 and 120, provisionally elements 119 and 120 would probably be discovered by 2022. named as Ununennium (Uue119) and (Ubn120) In the meantime, a team of researchers at the Joint Institute respectively have not yet been synthesised but efforts have gone for Nuclear Research in , Russia is planning to begin a long way and hopefully success may not take much longer. with new experiments on the synthesis of the element 119

The synthesis of these elements may push the limits of the using a target of Berkelium-249 (Bk249) and bombarding it with current technology due to the decreasing cross-sections of the Titanium–50 (Ti50). They have set the target to accomplish it production reactions and probably very short half-life periods by the end of the current year (2019). expected to be of the order of microseconds. The elements beyond 121, provisionally named as Element 120 Unbiunium (Ubu121) will likely be short-lived to be detected Similarly, the element with atomic number 120, provisionally

16 | Science Reporter | May 2019 Table 2. Predicted properties of elements 119-122

Element 119 Element 120 Element 121 Element 122 Provisional name Ununennium Unbinilium Unbiunium Unbibium Alternative name Eka-francium Eka- Eka-actinum Eka- Symbol Uue Ubn Ubu Ubb Mass number 315 299 320 - (most stable iso- tope) 1(Alkali ) 2(Alkaline earth met- 3(Super ) 4(Super actinide) al ) s s d g Electronic configu- [ Og]8s1 [ Og] 8s2 [ Og] 8s2p1 [ Og] 7d18s2 8p1 ration Oxidation states +1,+3 +1,+2,+4 +1,+3 +4 Electro-negativity 0.86 0.9 - - in Pauling scale 463.10 563.30 429 545 (1st) in Kj/mol in 273-303 953 - - degree K in de- 903 1973 - - gree K Heat of fusion in Kj/ 2.01-2.05 8.03-8.58 - - mol in g/cm3 3 7 - - Half life period of Uue (294)-10 Uue Ubn(299-) 3.7 Ubu(299)-1 Ubu(300)- - isotopes in micro (295)-20 Uue (296)- 1 Ubu(301)-1 seconds 10 Decay mode Alfa (for all 3) Alfa Alfa for all 3 - Decay products Ts(290), Ts(291), Og(295) Uue (295) Uue (296) - Ts(292) Uue (297)

named as Unbinilium (Ubn120), has not yet been synthesised, The first attempt to synthesise element 119 or Ununennium (Uue119) was made in 1985 at the super despite multiple attempts from German and Russian scientists. HILAC accelerator, Berkeley, California In 2007, a team at the Joint Institute for Nuclear Research (JINR), in Dubna (Russia) started experiments to create Ubn

from the nuclei of Iron 58 (Fe58) and Plutonium 244 (Pu244) but no atom was produced. In the same year, it was closely followed by another team of researchers at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, who used Uranium-238

(U238) and -64 (Ni64). Then in 2011, another team at

the GSI tried it with Californium-249 (Cf249) and Titanium-50 120 (Ti50). Although it was unable to synthesise Ubn it could suggest that the existing technology was not enough for the synthesis. So, the team has planned to begin new experiments

with a mixture of isotopes Californium-249 (Cf249) and

Californium-251 (Cf251) targeting those with Titanium-50

(Ti50) by 2020.

May 2019 | Science Reporter | 17 http://www.bbc.com/earth/story/20160115-how-many- more-chemical-elements-are-there-for-us-to-find

In the meantime, teams of scientists of the RIKEN, Japan Now, the question is: To what extent the Periodic Table and at GANIL in France have renewed their efforts to synthesise can be expanded? Many scientists believe that as the so-call Ubn120 by the end of 2020. While the Japanese team plans to “” in it, in which the elements have “closed use Curium 248 (Cm248) target and bombard it with Chromium nuclear structures” and have relatively more stability can be

(Cr54), the French team has chosen Plutonium (Pu244) and extended up to the elements 124 or 126, it may not be possible

Curium (Cm248) targets. to synthesise elements with higher mass numbers, as those may be extremely unstable. Elements 121 and 122 On the other hand, some physicists like Richard Feynman 121 The element 121, provisionally named Unbiunium (Ubu ), is are hopeful that it may go up to 137. According to them, when only three elements away from the heaviest known element – the atomic nuclei get larger and larger, the electrons have to 118 118 or Oganesson (Og ). Since the scientists are hopeful of go faster and faster. That puts a limitation on synthesising synthesising the two elements in between 119 and 120 quite elements beyond it. soon, naturally their next focus would 121, which is expected But, some others, by theoretical calculations, set the limit to be one of the last few reachable elements that can be at the element 170 or 173. They have even been provisionally synthesised with the currently available technology. (The limit named. It has also been predicted that while the element 118, may be anywhere between the elements with atomic numbers 119 and 120 are , and 120 and 124 or 126.) respectively, those from 121 to 157 will be super actinides, 158 The research team at RIKEN, Japan has already planned to 165, post-transition , 166-171, post super transition to attempt its synthesis by 2020 or 2021. It is expected to be metals, 172, a noble gas and 173 an alkali metal. in group 3 of the Periodic Table as the first super actinide and the 3rd element in the 8th period. However, it is almost agreed by all that with the presently On the other hand, there are no apparent plans currently available technology it may not be possible to go beyond the to synthesise the element 122, which has been provisionally element 124 or at the best, 126. So, for any foreseeable future named Unbibium (Ubb122). However, some researchers at the that may be the limit to the expansion of the Periodic Table. University of Jerusalem have claimed to have discovered it in 2008, in natural samples of Thorium, but the claim was 122 Prof. Ramesh Chandra Parida is Retired Professor of Chemistry, dismissed later. In the Periodic Table Ubb is expected to Odisha University of Agriculture and Technology. Address: Usha 121 nd follow Ubu as the 2 element of the super actinides and be Nivas, 124/2445, Khandagiri Vihar, Bhubaneswar-751030. the 4th in the period 8. Email: [email protected]

18 | Science Reporter | May 2019

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