14 | Science Reporter | May 2019

COVER STORY

Ramesh Chandra Parida

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

The Race for New Chemical Elements

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

Periodic Table of the Elements

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SYSTEMATISATION is an essential part of the scientific knowledge that makes the study of science easier and guides it into the future. Many eminent scientists have

made their epoch-making contributions to achieve it in various fields of science. Undoubtedly one of the foremost among them is the Russian Chemist Dmitri Ivanovich Mendeleev, who designed the “Periodic Table of Elements” a century and half ago (on 17 February 1869) making the study of chemistry systematic.

In fact, the process began with Dobeveiner. He classified certain elements with similar properties into groups of three, called triads. Then Newland (1863) observed that if the elements 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, like the 8th note of music and he called it the Law of Octaves.

Then Lothar Meyer (1869) plotted a graph of atomic volumes versus atomic weights of different elements and found that similar elements occupied similar positions on the curve. The same year Mendeleev proposed his famous periodic law, which stated: “Properties of elements are a periodic function of their atomic weights; similar elements are repeated at regular periods or intervals.”

He arranged the 63 elements, known at that time, in the ascending order of their atomic weights in a table consisting of 9 vertical columns, called groups and 7 horizontal rows called periods. In a period, elements exhibited gradual variation in their properties or in other words, periodicity of their behaviours. In order to maintain such conditions, Mendeleev had to leave certain places vacant in it, which he predicted would be filled by new elements to be discovered later. The table was called “Mendeleev’s Periodic Table”.

Modern Periodic TableDuring the days of Mendeleev the atomic number that indicates the number of electrons (or protons) present in an element and their arrangement in different orbitals, which actually determines its chemical and physical properties, was unknown. Therefore, in 1913 Henry Moseley changed the basis of classification of the elements replacing their atomic weights by atomic numbers, which formed the “Modern Periodic Table”.

It retained many of the characteristics of its predecessor and also had vacant positions, which were gradually filled up with newly discovered elements like Gallium, Scandium, Germanium, etc. And thus the process continued. And so, at present the Modern Periodic Table has been expanded to 18 groups and accommodates 118 elements of which the first 92 from Hydrogen (H1) to Uranium (U92) occur in nature and the rest have been synthesised.

The first man-made element Neptunium (Np93) was synthesised in 1940 and then gradually the list elongated to Oganesson (Og118), which has led to the completion of the 7th period of the periodic table (Table 1).

Scientists have, however, not stopped there yet. They are in the process of synthesising elements with atomic numbers from 119 to 122 and proposing to go beyond those, so that an additional new period (8th) can be added to it.

Atomic number Name Symbol

Year of discovery

93 Neptunium Np 1940

94 Plutonium Pu 1940-41

95 Americium Am 1944-45

96 Curium Cm 1944

97 Berkelium Bk 1949

98 Californium Cf 1950

99 Einsteinium Es 1952

100 Fermium Fm 1953

101 Mendelevium Md 1955

102 Nobelium No 1958

103 Lawrencium Lr 1961

104 Rutherfordium Rf 1965

105 Dubnium Db 1967

106 Seaborgium Sg 1972

107 Bohrium Bh 1981

108 Hassium Hs 1982

109 Meitnerium Mt 1984

110 Darmstadtium Ds 1994

111 Roentgenium Rg 1994

112 Copernicium Cn 1996

113 Nihonium Nh 2004

114 Flerorvium Fl 1998

115 Moscorium Mc 2004

116 Livermorium Lv 1998

117 Tennessine Ts 2010

118 Oganesson Og 1998

Table 1. List of man-made elements, their symbols and the year of discovery

So far essentially all the elements after Uranium (U92) have been synthesised at four laboratories: the Lawrence Berkeley National Laboratory in the US (elements 93 to 101 and jointly 103 to 105); the Joint Institute for Nuclear Research in Russia (elements 102, 114 to 118 and jointly 103 to 105); the GSI Helmholtz Centre for Heavy Ion Research in Germany (elements 107 to 112) and the RIKEN in Japan (element 113). All these have been named by the International Union of Pure and Applied Chemists (IUPAC).

Leap into FutureAfter the synthesis of Og118 the 7th period of the periodic table

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was completed. However, at present attempts are underway to synthesise the next higher elements with atomic numbers 119 to 124. By now scientists have come very close to achieving their objectives and have already predicted their properties (Table 2). This has begun the addition of a new period, the 8th, in the Periodic Table and has opened up the challenge to synthesise the next 14 elements to complete it.

The super heavy elements are synthesised by nuclear fusion – “hot” and “cold”. In hot fusion reactions, very light, high energy projectiles are accelerated towards very heavy targets (like the actinides) giving rise to compound nuclei at higher excitation energy (40 to 50 MeV) that may undergo fission or alternatively, evaporate several (3 to 5) neutrons.

On the other hand, cold fusion reactions use heavy projectiles, usually from the 4th period and lighter targets like Lead or Bismuth. The fused nuclei thus produced have relatively low excitation energy (10 to 20 MeV), which decreases the probability that these products will undergo fission reactions. Therefore, hot fusion reactions tend to produce more neutron-rich products, because the actinides have the highest neutron-to-proton ratios of any elements that can be made in microscopic quantities.

Elements with atomic numbers 119 and 120, provisionally named as Ununennium (Uue119) and Unbinilium (Ubn120) respectively have not yet been synthesised but efforts have gone a long way and hopefully success may not take much longer. The synthesis of these elements may push the limits of the current technology due to the decreasing cross-sections of the production reactions and probably very short half-life periods expected to be of the order of microseconds.

The elements beyond 121, provisionally named as Unbiunium (Ubu121) will likely be short-lived to be detected

properly with the currently available technology, decaying within a microsecond, before reaching the detectors. Therefore the possibility of detection and study of the elements with atomic numbers from 121 to 124 will depend greatly on the improvement of the technology and the theoretical model being used.

Element 119The first attempt to synthesise the element 119 or Ununennium (Uue119) was made in 1985 at the super HILAC accelerator, Berkeley, California, by bombarding a target of Einsteinium with mass number 254 (Es254) with Calcium 48 (Ca48) ions, but no atom was detected. Then several experiments were conducted in Russia until 2011, without much success.

A year after, in 2012 attempts to synthesis its isotopes with mass numbers 295 (Uue295) and 296 (Uue296) was made at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany by bombarding a target of Berkelium-249 (Bk249) with Titanium-50 (Ti50). Finally, in December 2017, the RIKEN Laboratories in Japan began the renewed search for element 119 by bombarding Curium-248 (Cm248) with Vanadium-51 (V51) beam and based on their results, it was predicted that both the elements 119 and 120 would probably be discovered by 2022.

In the meantime, a team of researchers at the Joint Institute for Nuclear Research in Dubna, Russia is planning to begin with new experiments on the synthesis of the element 119 using a target of Berkelium-249 (Bk249) and bombarding it with Titanium–50 (Ti50). They have set the target to accomplish it by the end of the current year (2019).

Element 120Similarly, the element with atomic number 120, provisionally

The first attempt to synthesise element 119 or Ununennium (Uue119) was made in 1985 at the super HILAC accelerator, Berkeley, California

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

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Element 119 Element 120 Element 121 Element 122

Provisional name Ununennium Unbinilium Unbiunium Unbibium

Alternative name Eka-francium Eka-radium Eka-actinum Eka-thorium

Symbol Uue Ubn Ubu Ubb

Mass number (most stable iso-tope)

315 299 320 -

Group 1(Alkali metal) 2(Alkaline earth met-al )

3(Super actinide) 4(Super actinide)

Block s s d g

Electronic configu-ration

[ Og]8s1 [ Og] 8s2 [ Og] 8s2p1 [ Og] 7d18s2 8p1

Oxidation states +1,+3 +1,+2,+4 +1,+3 +4

Electro-negativity in Pauling scale

0.86 0.9 - -

Ionization energy (1st) in Kj/mol

463.10 563.30 429 545

Melting point in degree K

273-303 953 - -

Boiling point in de-gree K

903 1973 - -

Heat of fusion in Kj/mol

2.01-2.05 8.03-8.58 - -

Density in g/cm3 3 7 - -

Half life period of isotopes in micro seconds

Uue (294)-10 Uue (295)-20 Uue (296)-10

Ubn(299-) 3.7 Ubu(299)-1 Ubu(300)-1 Ubu(301)-1

-

Decay mode Alfa (for all 3) Alfa Alfa for all 3 -

Decay products Ts(290), Ts(291), Ts(292)

Og(295) Uue (295) Uue (296) Uue (297)

-

Table 2. Predicted properties of elements 119-122

named as Unbinilium (Ubn120), has not yet been synthesised, despite multiple attempts from German and Russian scientists. 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 Nickel-64 (Ni64). Then in 2011, another team at the GSI tried it with Californium-249 (Cf249) and Titanium-50 (Ti50). Although it was unable to synthesise Ubn120 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.

The first attempt to synthesise element 119 or Ununennium (Uue119) was made in 1985 at the super HILAC accelerator, Berkeley, California

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In the meantime, teams of scientists of the RIKEN, Japan and at GANIL in France have renewed their efforts to synthesise Ubn120 by the end of 2020. While the Japanese team plans to use Curium 248 (Cm248) target and bombard it with Chromium (Cr54), the French team has chosen Plutonium (Pu244) and Curium (Cm248) targets.

Elements 121 and 122The element 121, provisionally named Unbiunium (Ubu121), is only three elements away from the heaviest known element – 118 or Oganesson (Og118). Since the scientists are hopeful of synthesising the two elements in between 119 and 120 quite soon, naturally their next focus would 121, which is expected to be one of the last few reachable elements that can be synthesised with the currently available technology. (The limit may be anywhere between the elements with atomic numbers 120 and 124 or 126.)

The research team at RIKEN, Japan has already planned to attempt its synthesis by 2020 or 2021. It is expected to be in group 3 of the Periodic Table as the first super actinide and the 3rd element in the 8th period.

On the other hand, there are no apparent plans currently to synthesise the element 122, which has been provisionally named Unbibium (Ubb122). However, some researchers at the University of Jerusalem have claimed to have discovered it in 2008, in natural samples of Thorium, but the claim was dismissed later. In the Periodic Table Ubb122 is expected to follow Ubu121 as the 2nd element of the super actinides and be the 4th in the period 8.

Now, the question is: To what extent the Periodic Table can be expanded? Many scientists believe that as the so-call “island of stability” in it, in which the elements have “closed nuclear structures” and have relatively more stability can be extended up to the elements 124 or 126, it may not be possible to synthesise elements with higher mass numbers, as those may be extremely unstable.

On the other hand, some physicists like Richard Feynman are hopeful that it may go up to 137. According to them, when the atomic nuclei get larger and larger, the electrons have to go faster and faster. That puts a limitation on synthesising elements beyond it.

But, some others, by theoretical calculations, set the limit at the element 170 or 173. They have even been provisionally named. It has also been predicted that while the element 118, 119 and 120 are noble gas, alkali metal and alkaline earth metal respectively, those from 121 to 157 will be super actinides, 158 to 165, post-transition metals, 166-171, post super transition metals, 172, a noble gas and 173 an alkali metal.

However, it is almost agreed by all that with the presently available technology it may not be possible to go beyond the element 124 or at the best, 126. So, for any foreseeable future that may be the limit to the expansion of the Periodic Table.

Prof. Ramesh Chandra Parida is Retired Professor of Chemistry, Odisha University of Agriculture and Technology. Address: Usha Nivas, 124/2445, Khandagiri Vihar, Bhubaneswar-751030. Email: paridanana47@gmail.com

http://www.bbc.com/earth/story/20160115-how-many-more-chemical-elements-are-there-for-us-to-find