Extension of the periodic table beyond the seventh period

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 were to be discovered, they would be placed in an eighth, and possibly ninth, period. The 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.^[1]

No elements in this region have been synthesized or discovered in nature. (Element 122 was claimed to exist naturally in April 2008, but this claim was widely believed to be erroneous.^[2]) The first element of the g-block may have atomic number 121, and this 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.

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.^[3]

Extended periodic table, including the g-block

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.

The positioning of the g-block in the table (to the left of the f-block, to the right, or in between) is speculative. The positions shown in the table above corresponds to the assumption that the Madelung rule will continue to hold at higher atomic number; this assumption may or may not be true. 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 122, the proximity of the electron shells makes placement in a simple table problematic; for example, calculations suggest that it may be elements 165 and 166 which occupy the 9s block (leaving the 8p orbital incomplete) assuming they are physically possible.^[5]

End of the periodic table

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The number of physically possible elements is unknown. The light-speed limit on electrons orbiting in ever-bigger electron shells theoretically limits neutral atoms to a Z of approximately 173,^[6] after which it would be nonsensical to assign the elements to blocks on the basis of electron configuration, while similar considerations of the nucleus limit ions to a Z of approximately 210.^{[citation needed]} (This range is grey and in italics in the table.) However, it is likely that the periodic table actually ends much earlier, possibly soon after the island of stability,^{[7][better ref needed]} which is expected to center around Z = 126.^[8]

Additionally the extension of the periodic and nuclides tables is restricted by the proton drip line and the neutron drip line.

Bohr model breakdown

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:

where Z is the atomic number, and α is the fine structure constant, a measure of the strength of electromagnetic interactions.^[9] Under this approximation, any element with an atomic number of greater than 137 would require 1s electrons to be traveling swifter than c, the speed of light. Hence a non-relativistic model such as the Bohr model is inadequate for such calculations.

The Dirac equation

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

where m₀ 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.^[10] Richard Feynman pointed out this effect, so the last element expected under this model, 137 (untriseptium), is sometimes called feynmanium.

However, a realistic calculation has to take into account the finite extension of the nuclear-charge distribution. This results in a critical Z of ≈ 173 (unseptrium), such that non-ionized atoms may be limited to elements equal to or lower than this.^[6]

See also

• Electron configuration
• Nuclear shell model
• Table of nuclides (combined)

References

• http://www.springerlink.com/content/j303171428652143/

External links

• Images of g-orbitals from the University of Kentucky
• jeries.rihani.com - The extended periodic table of the elements.