Extended periodic table

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 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 this region have been synthesized or discovered in nature. 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

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. However, if higher elements do exist, it is unlikely that they can be meaningfully assigned to the periodic table above approximately Z=173, as discussed in the next sections. This chart therefore ends at that number, without meaning to imply that all of those 173 elements are actually possible, nor to imply that heavier elements are not possible. (See also extended periodic table (large version).)

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.

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

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 correspond to the assumption that the Madelung rule will continue to hold for higher atomic numbers; 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 120, the proximity of the electron shells makes placement in a simple table problematic.

Pyykkö model

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. 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.

End of the periodic table

The number of physically possible elements is unknown. There is a theoretical limit for neutral atoms to a Z of approximately 173, after which it would be nonsensical to assign the elements to blocks on the basis of electron configuration. However, it is likely that the periodic table actually ends much earlier, possibly soon after the island of stability, which is expected to center around Z = 126.

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

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

$v=Z\alpha c\approx {\frac {Zc}{137.036}}$

where Z is the atomic number, and α is the fine structure constant, a measure of the strength of electromagnetic interactions. 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 semi-relativistic Dirac equation also has problems for Z > 137, for the ground state energy is

$E=m_{0}c^{2}{\sqrt {1-Z^{2}\alpha ^{2}}}$

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. Richard Feynman pointed out this effect, so the last element expected under this model, 137 (untriseptium), is sometimes called feynmanium (symbol: Fy).

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 (unsepttrium), such that neutral atoms may be limited to elements equal to or lower than this. Higher elements could only exist as ions, for example as salts.

See also

• Electron configuration
• Nuclear shell model
• Table of nuclides (combined)
• Period 8 element

External links

• Holler, Jim. "Images of g-orbitals". University of Kentucky.
• "The extended periodic table of the elements". {{cite web}}: |first= missing |last= (help)