Superactinide

Main articles: Period 8 element and Extended periodic table

Superactinides are the undiscovered chemical elements from atomic numbers 121 (unbiunium) until 153 (unpenttrium), at which the 5g and 6f electron shells are filled up. The superactinide series is predicted to follow the transactinide series and sits below it on the extended periodic table of the elements. The theoretical existence of the series was proposed by Glenn T. Seaborg, winner of the 1951 Nobel Prize in Chemistry. All superactinides are period 8 elements.

Expected properties

The first element of the g-block may have atomic number 121, and thus would have the systematic name unbiunium. Elements in this region are likely to be highly unstable with respect to radioactive decay, and have extremely short half lives, although element 126 is hypothesized to be within an island of stability that is resistant to fission but not to alpha decay. It is not clear how many elements beyond the expected island of stability are physically possible, or even if the superactindes are complete.

According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially-filled g-orbitals. However, spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number.

Elements

If superheavy elements continue follow the Aufbau principle, the superactinide series contains the following elements:

Superactinides

121 Ubu

122 Ubb

123 Ubt

124 Ubq

125 Ubp

126 Ubh

127 Ubs

128 Ubo

129 Ube

130 Utn

131 Utu

132 Utb

133 Utt

134 Utq

135 Utp

136 Uth

137 Uts

138 Uto

139 Ute

140 Uqn

141 Uqu

142 Uqb

143 Uqt

144 Uqq

145 Uqp

146 Uqh

147 Uqs

148 Uqo

149 Uqe

150 Upn

151 Upu

152 Upb

153 Upt

Blocks of the periodic table

s-block

p-block

d-block

f-block

g-block

(Undiscovered (theorized) elements are coloured in a lighter shade)

If the Pyykkö model is correct, the superactinide series contains the following elements instead:

Superactinides

121 Ubu

122 Ubb

123 Ubt

124 Ubq

125 Ubp

126 Ubh

127 Ubs

128 Ubo

129 Ube

130 Utn

131 Utu

132 Utb

133 Utt

134 Utq

135 Utp

136 Uth

137 Uts

138 Uto

141 Uqu

142 Uqb

143 Uqt

144 Uqq

145 Uqp

146 Uqh

147 Uqs

148 Uqo

149 Uqe

150 Upn

151 Upu

152 Upb

153 Upt

154 Upq

155 Upp

Blocks of the periodic table

s-block

p-block

d-block

f-block

g-block

(Undiscovered (theorized) elements are coloured in a lighter shade)

All of these hypothetical undiscovered elements are named by the International Union of Pure and Applied Chemistry (IUPAC) systematic element name standard which creates a generic name for use until the element has been discovered, confirmed, and an official name approved.

g-block

Attempts at synthesis

The only elements in this region of the periodic table that have had attempts to synthesise them are elements 122, 124 and 126.

The first attempt to synthesize unbibium was performed in 1972 by Flerov et al. at JINR, using the hot fusion reaction:

$\,^{238}_{92}\mathrm{U} + \,^{66}_{30}\mathrm{Zn} \to \,^{304}_{122}\mathrm{Ubb} ^{*} \to \ \mbox{no atoms}.$

No atoms were detected and a yield limit of 5 mb (5,000,000 pb) was measured. Current results (see ununquadium) have shown that the sensitivity of this experiment was too low by at least 6 orders of magnitude.

In 2000, the Gesellschaft für Schwerionenforschung performed a very similar experiment with much higher sensitivity:

$\,^{238}_{92}\mathrm{U} + \,^{70}_{30}\mathrm{Zn} \to \,^{308}_{122}\mathrm{Ubb} ^{*} \to \ \mbox{no atoms}.$

These results indicate that the synthesis of such heavier elements remains a significant challenge and further improvements of beam intensity and experimental efficiency is required. The sensitivity should be increased to 1fb.

Several experiments have been performed between 2000-2004 at the Flerov laboratory of Nuclear Reactions studying the fission characteristics of the compound nucleus ³⁰⁶Ubb. Two nuclear reactions have been used, namely²⁴⁸Cm+⁵⁸Fe and ²⁴²Pu+⁶⁴Ni. The results have revealed how nuclei such as this fission predominantly by expelling closed shell nuclei such as ¹³²Sn (Z=50, N=82). It was also found that the yield for the fusion-fission pathway was similar between ⁴⁸Ca and⁵⁸Fe projectiles, indicating a possible future use of ⁵⁸Fe projectiles in superheavy element formation.

On April 24, 2008, a group led by Amnon Marinov at the Hebrew University of Jerusalem claimed to have found single atoms of unbibium in naturally occurring thorium deposits at an abundance of between 10⁻¹¹and 10⁻¹², relative to thorium. The claim of Marinov et al. was criticized by a part of the scientific community, and Marinov says he has submitted the article to the journals Nature and Nature Physics but both turned it down without sending it for peer review.

A criticism of the technique, previously used in purportedly identifying lighter thorium isotopes by mass spectrometry, was published in Physical Review C in 2008. A rebuttal by the Marinov group was published in Physical Review C after the published comment.

A repeat of the thorium-experiment using the superior method of Accelerator Mass Spectrometry (AMS) failed to confirm the results, despite a 100-fold better sensitivity. This result throws considerable doubt on the results of the Marinov collaboration with regards to their claims of long-lived isotopes of thorium, roentgenium and unbibium.

In a series of experiments, scientists at GANIL have attempted to measure the direct and delayed fission of compound nuclei of elements with Z=114, 120, and 124 in order to probe shell effects in this region and to pinpoint the next spherical proton shell. In 2006, with full results published in 2008, the team provided results from a reaction involving the bombardment of a natural germanium target with uranium ions:

$\,^{238}_{92}\mathrm{U} + \,^{nat}_{32}\mathrm{Ge} \to \,^{308,310,311,312,314}\mathrm{Ubq} ^{*} \to \ fission.$

The team reported that they had been able to identify compound nuclei fissioning with half-lives > 10⁻¹⁸s. Although very short, the ability to measure such decays indicated a strong shell effect at Z=124. A similar phenomenon was found for Z=120 but not for Z=114.

The first attempt to synthesize unbihexium was performed in 1971 by Bimbot et al. using the hot fusion reaction:

$\,^{232}_{90}\mathrm{Th} + \,^{84}_{36}\mathrm{Kr} \to \,^{316}_{126}\mathrm{Ubh} ^{*} \to \ no \ atoms$

A high energy alpha particle was observed and taken as possible evidence for the synthesis of unbihexium. Recent research suggests that this is highly unlikely as the sensitivity of experiments performed in 1971 would have been several orders of magnitude too low according to current understanding. To date, no other attempt has been made to synthesize unbihexium.

Element 137

Untriseptium, element 137, is sometimes called feynmanium (symbol Fy) because Richard Feynman noted that a simplistic interpretation of the relativistic Dirac equation runs into problems with electron orbitals at Z > 1/α = 137, suggesting that neutral atoms cannot exist beyond untriseptium, and that a periodic table of elements based on electron orbitals therefore breaks down at this point. However, a more rigorous analysis calculates the limit to be Z ≈ 173.

Bohr model breakdown

The Bohr model exhibits difficulty for atoms with atomic number greater than 137, for the speed of an electron in a 1s electron orbital, v, is given by

$v = Z \alpha c \approx \frac{Z c}{137.036}$

where Z is the atomic number, and α is the fine structure constant, a measure of the strength of electromagnetic interactions. Under this approximation, any element with an atomic number of greater than 137 would require 1s electrons to be traveling faster than c, the speed of light. Hence the non-relativistic Bohr model is clearly inaccurate when applied to such an element.

The Dirac equation

The relativistic Dirac equation also has problems for Z > 137, for the ground state energy is

$E=m c^2 \sqrt{1-Z^2 \alpha^2}$

where m is the rest mass of the electron. For Z > 137, the wave function of the Dirac ground state is oscillatory, rather than bound, and there is no gap between the positive and negative energy spectra, as in the Klein paradox.

More accurate calculations including the effects of the finite size of the nucleus indicate that the binding energy first exceeds 2mc² for Z > Z_cr ≈ 173. ForZ > Z_cr, if the innermost orbital is not filled, the electric field of the nucleus will pull an electron out of the vacuum, resulting in the spontaneous emission of a positron.

More complete analysis involving relativity shows that the contradiction this particle poses may actually occur in the hypothetical element untriennium (Z = 139; see unsolved problems in chemistry).

f-block

The relativstic and quantum effects for the electron clouds of these elements are expected to be even greater than those for the g-block elements, because these elements have higher atomic number. If these elements could actually be observed, they would likely be observed to have similar chemical properties, but the effect of the closeness of the 5g and 6f (and possibly also the 7d and 8p) subshells is unclear and difficult to predict because of the relativistic and quantum effects. These orbitals, being so close in energy, may fill together all at the same time, resulting in a series of very similar elements with many barely distinguishable oxidation states. The basis of periodic trends based on electron configurations may thus no longer hold.

The existence of such atoms is probably theoretically possible as the upper limit for atomic number is likely Z = 173 due to the light-speed limit, after which assigning electron shells would be nonsensical and elements would only be able to exist as ions, but it is not clear if our technology will ever be enough to synthesise them.

d-block and p-block

Although element 153 would likely be taken to be the last superactinide based on previous periods, the electron configurations for the d-block and p-block period 8 elements would likely be nothing more than mathematical extrapolation because of the extreme quantum and relativistic effects the electron clouds will experience. In the unlikely case that their chemical properties may eventually be studied, it is likely that all existing classifications will be inadequate to describe them. Due to the breakdown of periodic trends expected in this region due to the closeness of energy of the 5g, 6f, 7d and 8p orbitals and other relativistic effects, it seems likely that the properties and placement in the periodic table of these elements may be of only formal significance.

Bibiliography

J. Huheey: Anorganische Chemie, 2. Auflage, 1995