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A period 9 element is any one of 50 hypothetical chemical elements (unhexennium through biunoctium) belonging to an eighth period of the periodic table of the elements. They may be referred to using IUPAC systematic element names. None of these elements have been synthesized, and it is possible that none have isotopes with stable enough nuclei to receive significant attention in the near future. It is also probable that, due to drip instabilities,none of the period 9 elements are physically possible and the periodic table may end soon after the island of stability at unbihexium with atomic number 126. Period 9 is likely to be the last period in the periodic table.

If it were possible to produce sufficient quantities of these elements that would allow the study of their chemistry, these elements may well behave very differently from those of previous periods. This is because their electronic configurations may be altered by quantum and relativistic effects, as the energy levels of the 6g, 7f and 8d orbitals are so close to each other that they may well exchange electrons with each other. This would result in a large number of elements in the superactinide series that would have extremely similar chemical properties that would be quite unrelated to elements of lower atomic number.

The names given to these unattested elements are all IUPAC systematic names.

History

There are currently seven periods in the periodic table of chemical elements, culminating with atomic number 118. If further elements with higher atomic numbers than this are discovered, they will be placed in additional periods (likely 8 and 9), laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned. Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to have an additional so-called g-block, containing 18 elements with partially filled g-orbitals in each period. An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969. While Seaborg's version of the extended period had the heavier elements following the pattern set by lighter elements, other models do not. Pekka Pyykkö, for example, used computer modeling to calculate the positions of elements up to Z = 172, and found that several were displaced from the Madelung rule.

Elements

Period 9 is divided into five blocks, and it is the second period that includes the g-block; however, spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number.

Attempts at synthesis

No synthesis has been attempted for period 9 elements.

Groups of elements

Period 9 is the second period to have g-block elements, which have atomic numbers from 171 onwards, but it is not clear when the filling of the 6g subshell ends. These elements belong to the chemical series of eka-superactinides, characterised by the filling of the 6g and 7f subshells, and they could therefore have different chemical properties that are reminiscent of the actinides; however, the proximity of the 6g and 7f subshells and the small gap between them and the 8d and 9p subshells could lead to a large number of elements whose properties are independent of their position in the periodic table.

The period 9 elements are likely to be too unstable to be detected.

Unsepttrium

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

${\displaystyle E=mc^{2}{\sqrt {1-Z^{2}\alpha ^{2}}}}$

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 2mc for Z > Z_cr ≈ 173. For Z > 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.

f-block elements

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 6g and 7f (and possibly also the 8d and 9p) 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 elements

Although element 203 would likely be taken to be the last eka-superactinide based on previous periods, the electron configurations for the d-block and p-block period 9 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 6g, 7f, 8d and 9p 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.

Characteristics

No period 9 element has been synthesized.

Chemical

Periodic trends may not continue to hold at such high atomic number, and in fact may already break down in the late seventh period. For example, chemical studies performed in 2007 indicate that flerovium may possess some non-eka-lead properties and may behave as the first superheavy element that portrays some noble-gas-like properties due to relativistic effects.

Physical and atomic

Isotopes

Isotopes of period 9 elements may contain more than 220 neutrons, such as Ust. Mass numbers will be in the 400s (and probably also the 500s).

Electron configurations

See also

• Extension of the periodic table beyond the seventh period
• Nucleon drip line

Notes

1. The heaviest element that has been synthesized to date is ununoctium with atomic number 118, which is the last period 7 element.

References

1. ^ Seaborg (ca. 2006). "transuranium element (chemical element)". Encyclopædia Britannica. Retrieved 2010-03-16. {{cite web}}: Check date values in: |date= (help)
2. ^ "Extended elements: new periodic table". 2010.
3. J.D. Bjorken, S.D. Drell (1964). Relativistic Quantum Mechanics. McGraw-Hill.
4. W. Greiner, S. Schramm (2008). "American Journal of Physics". 76: 509. {{cite journal}}: Cite journal requires |journal= (help), and references therein.
5. Walter Greiner and Stefan Schramm (2008). "Resource Letter QEDV-1: The QED vacuum". American Journal of Physics. 76 (6): 509. Bibcode:2008AmJPh..76..509G. doi:10.1119/1.2820395., and references therein.
6. Cite error: The named reference LBNL was invoked but never defined (see the help page).
7. Gas Phase Chemistry of Superheavy Elements, lecture by Heinz W. Gäggeler, Nov. 2007. Last accessed on Dec. 12, 2008.

• Periodic table