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A period 9 element is any one of 50 hypothetical chemical elements (unhexennium through biunoctium) belonging to a ninth period of the periodic table of the elements. This period is likely to be the last. 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.

Is period 9 possible?

If the lower period 9 elements are possible, unsepttrium may be the heaviest possible neutral element. If all period 9 elements are possible, element 200 might be in this period. However, the Pyykko Model lists elements 169-172 in period 8, and elements 165-168 in period 9.

Chemistry

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 eka-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.

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 proper ties 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. No elements in period 9 have been synthesized or discovered in nature. 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.

Period 9 in the periodic table (based on the guess of period 8-not the Pyykko model):

Synthesis

No synthesis has been attempted for period 9 elements.

Unsepttrium

Main article: Unsepttrium

Unsepttrium is likely the heaviest possible neutral element. Its atomic number is 173 and its atomic symbol is Ust.

=Significance

Although 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, a more rigorous analysis calculates the limit to be Z ≈ 173, meaning that neutral atoms most likely cannot exist with Z greater than 173. This makes unsepttrium theoretically the heaviest neutral element possible.

The Dirac equation

The relativistic Dirac equation 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. Although 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 taking into account the effects of the finite size of the nucleus indicate that the binding energy first exceeds 2mc for Z > Zcr ≈ 173. For Z > Zcr, 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.