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A period 9 element is any one of 8 hypothetical chemical elements (unhexpentium through unseptbium) belonging to a ninth 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 possible 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. The names given to these unattested elements are all IUPAC systematic names. In contrast to periods four through eight, the ninth period of the periodic table is expected to lack any transition elements, and is expected to be analogous to the second and third periods.

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, 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 contain elements with filled g-orbitals in their ground state. An eight-period table containing these elements was suggested by Glenn T. Seaborg in 1969. No elements in this region 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, as it did not take into account relativistic effects, models that take relativistic effects into account do not. Pekka Pyykkö and B. Fricke used computer modeling to calculate the positions of elements up to Z = 172 (comprising periods 8 and 9), and found that several were displaced from the Madelung rule. Fricke predicted the structure of the extended periodic table up to Z = 172 to be:

v t e Extended periodic table
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 ① H He ② Li Be B C N O F Ne ③ Na Mg Al Si P S Cl Ar ④ K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr ⑤ Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe ⑥ Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn ⑦ Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og ⑧ 119 120 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 ‍ ‍ 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142
s-block g-block f-block d-block p-block

Predicted properties

Chemical and physical properties

Period 9 should begin with elements 165 (unhexpentium) and 166 (unhexhexium), which should be normal alkali and alkaline earth metals. The 9s electrons should have ionization energies comparable to those of the 3s electrons of sodium and magnesium, due to relativistic effects causing the 9s electrons to be much more strongly bound than non-relativistic calculations would predict. Elements 165 and 166 should mostly exhibit the +1 and +2 oxidation states respectively; however, the ionization energies of the 7d electrons are low enough to possibly allow higher oxidation states like +3 and +4 to occur.

In elements 167 to 172, the 9p_1/2 and 8p_3/2 shells will be filled. Their energy eigenvalues are so close together that they behaves as one combined p shell, similar to the non-relativistic 2p and 3p shells. Thus, the inert pair effect does not occur and the most common oxidation states of elements 167 to 170 should be +3, +4, +5, and +6 respectively. Element 171 (unseptunium) is expected to be a halogen, showing various oxidation states ranging from –1 to +7. Its electron affinity should be 3.0 eV, allowing it to form a hydride, UsuH. Element 172 (unseptbium) should be a noble gas with chemical behaviour similar to that of xenon, as their ionization energies should be very similar. The only main difference between them is that element 172, unlike xenon, is expected to be a liquid or a solid due to its much higher atomic weight.

Some predicted properties of the period 9 elements. The metallic radii and densities are first approximations.

Property	165	166	167	168	169	170	171	172
Relative atomic mass
Group	1	2	13	14	15	16	17	18
Valence electron configuration	9s	9s	9s 9p	9s 9p	9s 9p 8p	9s 9p 8p	9s 9p 8p	9s 9p 8p
Stable oxidation states	1	2	3	4	5	6	−1, 3, 7	0, 4, 6, 8
First ionization energy	521.0 kJ/mol	627.2 kJ/mol	617.5 kJ/mol	723.6 kJ/mol	800.8 kJ/mol	887.7 kJ/mol	984.2 kJ/mol	1090.3 kJ/mol
Metallic or covalent radius	250 pm	200 pm	190 pm	180 pm	175 pm	170 pm	165 pm	220 pm
Density	7 g/cm	11 g/cm	17 g/cm	19 g/cm	18 g/cm	17 g/cm	16 g/cm	9 g/cm

Nuclear properties

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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. Emsley, John (2011). Nature's Building Blocks: An A-Z Guide to the Elements (New ed.). New York, NY: Oxford University Press. ISBN 978-0-19-960563-7.
2. Seaborg, Glenn (August 26, 1996). "An Early History of LBNL".
3. Frazier, K. (1978). "Superheavy Elements". Science News. 113 (15): 236–238. doi:10.2307/3963006. JSTOR 3963006.
4. "Extended elements: new periodic table". 2010.
5. ^ Fricke, B.; Greiner, W.; Waber, J. T. (1971). "The continuation of the periodic table up to Z = 172. The chemistry of superheavy elements". Theoretica chimica acta. 21 (3). Springer-Verlag: 235–260. doi:10.1007/BF01172015. Retrieved 28 November 2012.
6. ^ Haire, Richard G. (2006). "Transactinides and the future elements". In Morss; Edelstein, Norman M.; Fuger, Jean (eds.). The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Dordrecht, The Netherlands: Springer Science+Business Media. ISBN 1-4020-3555-1.{{cite book}}: CS1 maint: ref duplicates default (link)

v t e Extended periodic table
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 ① H He ② Li Be B C N O F Ne ③ Na Mg Al Si P S Cl Ar ④ K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr ⑤ Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe ⑥ Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn ⑦ Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og ⑧ 119 120 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 ‍ ‍ 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142
s-block g-block f-block d-block p-block

• Periodic table