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| There are currently seven [[period (periodic table)|period]]s in the [[periodic table]] of [[chemical elements]], culminating with [[atomic number]] [[Ununoctium|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 have an additional so-called '''g-block''', containing at least 18 elements with partially filled g-[[atomic orbital|orbital]]s in each period. An '''eight-period table''' containing this block was suggested by [[Glenn T. Seaborg]] in 1969.<ref>{{cite web|url=http://www.lbl.gov/LBL-PID/Nobelists/Seaborg/65th-anniv/29.html|title= An Early History of LBNL|first=Glenn|last=Seaborg|date=August 26, 1996}}</ref><ref>{{cite journal | doi = 10.2307/3963006 | last1 = Frazier | first1 = K. | title = Superheavy Elements | journal = Science News | volume = 113 | issue = 15 | pages = 236–238 | year = 1978 | jstor = 3963006}}</ref>
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| No elements in this region have been synthesized or discovered in nature.<ref>[[Unbibium|Element 122]] was claimed to exist naturally in April 2008, but this claim was widely believed to be erroneous. {{cite web|url=http://www.rsc.org/chemistryworld/News/2008/May/02050802.asp|title=Heaviest element claim criticised|publisher=Rsc.org|date=2008-05-02|accessdate=2010-03-16}}</ref> The first element of the g-block may have [[atomic number]] 121, and thus would have the [[Systematic element name|systematic name]] [[unbiunium]]. Elements in this region are likely to be highly unstable with respect to [[radioactive decay]], and have extremely short [[half life|half lives]], although [[unbihexium|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, if period 8 is complete, or if there is a period 9.
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| According to the orbital approximation in [[quantum mechanics|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]].
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| ==Extended periodic table, including the g-block==
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| It is unknown how far the periodic table might extend beyond the known 118 elements. [[Glenn T. Seaborg]] suggested that the highest possible element may be under ''Z''=130.<ref name=EB/> However, Walter Greiner predicts that there may not be a highest possible element.<ref name="rsc"/> (See also [[extended periodic table (large version)]].)
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| {{Compact extended periodic table}}
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| 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. However, typically they are not even named at all in the scientific literature, and are simply referred to by their atomic numbers; hence, element 164 would usually not be called "unhexquadium" (the IUPAC systematic name), but rather "element 164" with symbol "164", "(164)", or "E164".
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| {{As of|2011|04}}, synthesis has been attempted for only [[ununennium]], [[unbinilium]], [[unbibium]], [[unbiquadium]], [[unbihexium]], and [[unbiseptium]]. (Z = 119, 120, 122, 124, 126, and 127)
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| At element 118, the orbitals 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 5f, 6s, 6p, 6d, 7s and 7p are assumed to be filled, with the remaining orbitals unfilled. The orbitals of the eighth period are predicted to be filled in the order 8s, 5g, 6f, 7d, 8p. However, after approximately element 120, the proximity of the electron shells makes placement in a simple table problematic.
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| ==Pyykkö model==
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| {{Expert-subject|date=December 2011}}
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| Not all models show the higher elements following the pattern established by lighter elements. [[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 [[Aufbau principle|Madelung energy-ordering rule]].<ref>{{Cite web|url=http://www.rsc.org/Publishing/ChemScience/Volume/2010/11/Extended_elements.asp|title=Extended elements: new periodic table|year=2010}}</ref> He predicts that the orbital shells will fill up in this order:
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| *8s,
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| *5g,
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| *the first two spaces of 8p,
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| *6f,
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| *7d,
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| *9s,
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| *the first two spaces of 9p,
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| *the rest of 8p.
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| He also suggests that period 8 be split into three parts:
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| *8a, containing 8s,
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| *8b, containing the first two elements of 8p,
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| *8c, containing 7d and the rest of 8p.<ref name="PT172">{{Cite journal|last1=Pyykkö|first1=Pekka|title=A suggested periodic table up to Z≤ 172, based on Dirac–Fock calculations on atoms and ions|journal=Physical Chemistry Chemical Physics|volume=13|issue=1|pages=161–8|year=2011|pmid=20967377|doi=10.1039/c0cp01575j|bibcode = 2011PCCP...13..161P }}</ref>
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| Fricke ''et al.'' also predicted the extended periodic table up to 172.<ref>{{Cite journal|last1=Fricke|first1=B|last2=Greiner|first2=W|last3=Waber|first3=J.T| title=The continuation of the periodic table up to Z = 172. The chemistry of superheavy elements| journal=Theoretica chimica acta |volume=21|issue=3|pages=235–260|year=1971|doi=10.1007/BF01172015}}</ref> This model has been more widely used among scientists and is shown above as the main form of the extended periodic table.
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| {{Wide template|Periodic table (Pyykkö model)}}
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| ==End of the periodic table==
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| {{Expert-subject|physics|date=August 2009}}
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| The number of physically possible elements is unknown. A low estimate is that the periodic table may end soon after the [[island of stability]],<ref name=EB>{{cite web|author=Seaborg|url=http://www.britannica.com/EBchecked/topic/603220/transuranium-element|title=transuranium element (chemical element)|publisher=Encyclopædia Britannica|date=c. 2006|accessdate=2010-03-16}}</ref> which is expected to center around ''Z'' = 126, as the extension of the periodic and nuclides tables is restricted by the proton and the neutron [[Nuclear drip line|drip lines]];<ref>{{cite journal | first1=S. |last1=Cwiok|first2= P.-H.|last2= Heenen |first3= W.|last3= Nazarewicz |year=2005|title=Shape coexistence and triaxiality in the superheavy nuclei|journal=Nature|volume=433|bibcode = 2005Natur.433..705C |doi = 10.1038/nature03336 | issue=7027 | pmid=15716943 | pages=705–9}}</ref> however, some, such as Walter Greiner, predict that there may not be an end to the periodic table at all.<ref name="rsc">{{cite web |url=http://www.rsc.org/chemistryworld/Issues/2010/November/ColumnThecrucible.asp |title=Would element 137 really spell the end of the periodic table? Philip Ball examines the evidence|author=Philip Ball |date=November 2010 |work=[[Chemistry World]]|publisher=[[Royal Society of Chemistry]] |accessdate=2012-09-30}}</ref> Other predictions of an end to the periodic table include ''Z'' = 128 ([[John Emsley]]) and ''Z'' = 155 (Albert Khazan).<ref name="emsley">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks: An A-Z Guide to the Elements|edition=New|year=2011|publisher=Oxford University Press|location=New York, NY|isbn=978-0-19-960563-7|page=593}}</ref>
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| ==Feynmanium and elements above the atomic number 137==
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| [[Richard Feynman]] noted<ref>
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| {{cite web
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| |first=G.|last= Elert
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| |date=
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| |title=Atomic Models
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| |url=http://physics.info/atomic-models/
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| |work=The Physics Hypertextbook
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| |accessdate=2009-10-09
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| }}</ref> that a simplistic interpretation of the [[Theory of relativity|relativistic]] [[Dirac equation]] runs into problems with electron orbitals at ''Z'' > 1/α ≈ 137 as described in the sections below, 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. On the other hand, a more rigorous analysis{{by whom|date=April 2013}} calculates the limit to be ''Z'' ≈ 173, and also that this limit would not actually spell the end of the periodic table.<ref name="rsc"/>
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| ===Bohr model===
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| The [[Bohr model]] exhibits difficulty for atoms with atomic number greater than 137, for the speed of an electron in a [[Atomic orbital|1s electron orbital]], ''v'', is given by
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| :<math>v = Z \alpha c \approx \frac{Z c}{137.036}</math> | |
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| where ''Z'' is the [[atomic number]], and ''α'' is the [[fine structure constant]], a measure of the strength of electromagnetic interactions.<ref>{{cite book
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| |first1=R. |last1=Eisberg|first2= R.|last2= Resnick
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| |year=1985
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| |title=Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles
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| |publisher=[[John Wiley & Sons|Wiley]]
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| |isbn=
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| }}</ref> 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.
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| ===Relativistic Dirac equation===
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| The [[Theory of relativity|relativistic]] [[Dirac equation]] has problems for ''Z'' > 137, for the ground state energy is
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| :<math>E=\frac{m c^2}{\sqrt{1-\frac{v^2}{c^2}}}=\frac{m c^2}{\sqrt{1-Z^2 \alpha^2}}</math>
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| 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]].<ref>
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| {{cite book
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| |first1=J. D.|last1= Bjorken|first2=S. D.|last2= Drell
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| |year=1964
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| |title=Relativistic Quantum Mechanics
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| |publisher=[[McGraw-Hill]]
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| |isbn=
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| }}</ref> More accurate calculations taking into account the effects of the finite size of the nucleus indicate that the binding energy first exceeds 2''mc''<sup>2</sup> for ''Z'' > ''Z''<sub>cr</sub> ≈ 173. For ''Z'' > ''Z''<sub>cr</sub>, if the innermost orbital (1s) is not filled, the electric field of the nucleus will [[pair production|pull an electron out of the vacuum]], resulting in the spontaneous emission of a [[positron]].<ref>
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| {{cite journal
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| |first1=W. |last1=Greiner|first2= S. |last2=Schramm
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| |year=2008
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| |title=[[American Journal of Physics]]
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| |volume=76 |pages=509
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| |doi=
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| }}, and references therein</ref> However, this does not happen if the innermost orbital is filled, so that ''Z'' = 173 does not constitute a limit to the periodic table.<ref name="rsc"/>
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| ==See also==
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| * [[Electron configuration]]
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| * [[Nuclear shell model]]
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| * [[Table of nuclides (combined)]]
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| * [[Period 8 element]]
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| * [[Period 9 element]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| * {{cite web|url=http://www.uky.edu/~holler/html/g.html|title= Images of g-orbitals|publisher=University of Kentucky|first=Jim|last= Holler}}
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| * {{Cite web|url=http://jeries.rihani.com |title= The extended periodic table of the elements| first = Jeries}}
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| * {{Cite web|url=http://www.ericscerri.com |title= Eric Scerri's website for the elements and the periodic table}}
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| *
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| {{Periodic table footer}}
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| {{Compact extended periodic table}}
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| [[Category:Periodic table]]
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| [[Category:Hypothetical chemical elements]]
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