Extended periodic table

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An extended periodic table theorises about elements beyond element 118 (the last one of period 7). Currently seven periods in the periodic table of chemical elements are known and proven, 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 have an additional so-called g-block, containing at least 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.[1][2]

No elements in this region have been synthesized or discovered in nature.[3] 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, if period 8 is complete, or if there is a period 9.

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.

Extended periodic table, including the g-block[edit]

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.[4] However, Walter Greiner predicts that there may not be a highest possible element.[5]

Legend
Alkali metal Alkaline earth metal Super­actinide Eka-​super­actinide Lan­thanide Actinide Transition metal Other metal Metalloid Polyatomic nonmetal Diatomic nonmetal Noble gas
predicted predicted predicted predicted     predicted predicted predicted predicted predicted predicted

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".

As of April 2011, synthesis has been attempted for only ununennium, unbinilium, unbibium, unbiquadium, unbihexium, and unbiseptium. (Z = 119, 120, 122, 124, 126, and 127)

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.

Pyykkö model[edit]

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 Madelung energy-ordering rule.[6] He predicts that the orbital shells will fill up in this order:

  • 8s,
  • 5g,
  • the first two spaces of 8p,
  • 6f,
  • 7d,
  • 9s,
  • the first two spaces of 9p,
  • the rest of 8p.

He also suggests that period 8 be split into three parts:

  • 8a, containing 8s,
  • 8b, containing the first two elements of 8p,
  • 8c, containing 7d and the rest of 8p.[7]

Fricke et al. also predicted the extended periodic table up to 172.[8] This model has been more widely used among scientists and is shown above as the main form of the extended periodic table.

Pyykkö model
Displaced elements are in boldface
 8  119
Uue
120
Ubn
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
156
Uph
157
Ups
158
Upo
159
Upe
160
Uhn
161
Uhu
162
Uhb
163
Uht
164
Uhq
139
Ute
140
Uqn
169
Uhe
170
Usn
171
Usu
172
Usb
9 165
Uhp
166
Uhh
167
Uhs
168
Uho
  s-block g-block f-block d-block p-block

End of the periodic table[edit]

The number of physically possible elements is unknown. A low estimate is that the periodic table may end soon after the island of stability,[4] 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 drip lines;[9] however, some, such as Walter Greiner, predict that there may not be an end to the periodic table at all.[5] Other predictions of an end to the periodic table include Z = 128 (John Emsley) and Z = 155 (Albert Khazan).[10]

Feynmanium and elements above the atomic number 137[edit]

Richard Feynman noted[11] that a simplistic interpretation of the 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 calculates the limit to be Z ≈ 173, and also that this limit would not actually spell the end of the periodic table.

Bohr model[edit]

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.[12] 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.

Relativistic Dirac equation[edit]

The relativistic Dirac equation gives the ground state energy as

E=\frac{m c^2}{\sqrt{1-\frac{v^2}{c^2}}}=\frac{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.[13] More accurate calculations taking into account the effects of the finite size of the nucleus indicate that the binding energy first exceeds 2mc2 for Z > Zcr ≈ 173. For Z > Zcr, if the innermost orbital (1s) 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.[14] However, this does not happen if the innermost orbital is filled, so that Z = 173 does not constitute a limit to the periodic table.[5]

See also[edit]

References[edit]

  1. ^ Seaborg, Glenn T. (August 26, 1996). "An Early History of LBNL". 
  2. ^ Frazier, K. (1978). "Superheavy Elements". Science News 113 (15): 236–238. doi:10.2307/3963006. JSTOR 3963006. 
  3. ^ Element 122 was claimed to exist naturally in April 2008, but this claim was widely believed to be erroneous. "Heaviest element claim criticised". Rsc.org. 2008-05-02. Retrieved 2010-03-16. 
  4. ^ a b Seaborg, Glenn T. (c. 2006). "transuranium element (chemical element)". Encyclopædia Britannica. Retrieved 2010-03-16. 
  5. ^ a b c Philip Ball (November 2010). "Would element 137 really spell the end of the periodic table? Philip Ball examines the evidence". Chemistry World. Royal Society of Chemistry. Retrieved 2012-09-30. 
  6. ^ "Extended elements: new periodic table". 2010. 
  7. ^ Pyykkö, Pekka (2011). "A suggested periodic table up to Z≤ 172, based on Dirac–Fock calculations on atoms and ions". Physical Chemistry Chemical Physics 13 (1): 161–8. Bibcode:2011PCCP...13..161P. doi:10.1039/c0cp01575j. PMID 20967377. 
  8. ^ 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): 235–260. doi:10.1007/BF01172015. 
  9. ^ Cwiok, S.; Heenen, P.-H.; Nazarewicz, W. (2005). "Shape coexistence and triaxiality in the superheavy nuclei". Nature 433 (7027): 705–9. Bibcode:2005Natur.433..705C. doi:10.1038/nature03336. PMID 15716943. 
  10. ^ Emsley, John (2011). Nature's Building Blocks: An A-Z Guide to the Elements (New ed.). New York, NY: Oxford University Press. p. 593. ISBN 978-0-19-960563-7. 
  11. ^ Elert, G. "Atomic Models". The Physics Hypertextbook. Retrieved 2009-10-09. 
  12. ^ Eisberg, R.; Resnick, R. (1985). Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles. Wiley. 
  13. ^ Bjorken, J. D.; Drell, S. D. (1964). Relativistic Quantum Mechanics. McGraw-Hill. 
  14. ^ Greiner, W.; Schramm, S. (2008). American Journal of Physics 76. p. 509. , and references therein

External links[edit]

Legend
Alkali metal Alkaline earth metal Super­actinide Eka-​super­actinide Lan­thanide Actinide Transition metal Other metal Metalloid Polyatomic nonmetal Diatomic nonmetal Noble gas
predicted predicted predicted predicted     predicted predicted predicted predicted predicted predicted