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There is great debate in the scientific community over where lanthanides and actinides should be placed within the standard periodic table. There is no universally agreed upon standard for this, and different mediums will often choose different options. Each of the four main alternatives are justifiable, though there are pros and cons to each as well.

Why is there not one option?

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Main alternatives

Scandium and yttrium are always classified as group 3 elements, but it is controversial which elements should follow them in group 3, lanthanum and actinium or lutetium and lawrencium. The current IUPAC definition of the term "lanthanoid" includes fifteen elements including both lanthanum and lutetium, and that of "transition element" applies to lanthanum and actinium, as well as lutetium but not lawrencium, since it does not correctly follow the Aufbau principle. Normally, the 103rd electron would enter the d-subshell, but quantum mechanical research has found that the configuration is actually 7s5f7p due to relativistic effects. IUPAC thus has not recommended a specific format for the in-line-f-block periodic table, leaving the dispute open.

• Lanthanum and actinium are sometimes considered the remaining members of group 3. In their most commonly encountered tripositive ion forms, these elements do not possess any partially filled f-orbitals, thus continuing the scandium—yttrium—lanthanum—actinium trend, in which all the elements have relationship similar to that of elements of the calcium—strontium—barium—radium series, the elements' left neighbors in s-block. However, different behavior is observed in other d-block groups, especially in group 4, in which zirconium, hafnium and rutherfordium share similar chemical properties lacking a clear trend.

• In other tables, lutetium and lawrencium are classified as the remaining members of group 3. In these tables, lutetium and lawrencium end (or sometimes proceed) the lanthanide and actinide series, respectively. Since the f-shell is nominally full in the ground state electron configuration for both of these metals, they behave most similarly to other period 6 and period 7 transition metals compared to the other lanthanides and actinides, and thus logically exhibit properties similar to those of scandium and yttrium. (This behavior is expected for lawrencium, but has not been observed because sufficient quantities of lawrencium have not yet been synthesized.)

• Some tables, including the official IUPAC table refer to all lanthanides and actinides by a marker in group 3. This sometimes is believed to be the inclusion of all 30 lanthanide and actinide elements as included in group 3. Lanthanides, as electropositive trivalent metals, all have a closely related chemistry, and all show many similarities to scandium and yttrium, but they also show additional properties characteristic of their partially filled f-orbitals which are not common to scandium and yttrium.

• Exclusion of all elements is based on properties of earlier actinides, which show a much wider variety of chemistry (for instance, in range of oxidation states) within their series than the lanthanides, and comparisons to scandium and yttrium are even less useful. However, these elements are destabilized, and if they were stabilized to more closely match chemistry laws, they would be similar to lanthanides as well. Also, the later actinides from californium onwards behave more like the corresponding lanthanides, with only the valence +3 (and sometimes +2) shown.

Lanthanide and actinides' placement in other periodic tables

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Notes

1. The expected configuration of lawrencium if it did obey the Aufbau principle would be 7s5f6d, with the normal incomplete 6d-subshell in the neutral state.

References

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "transition element". doi:10.1351/goldbook.T06456
2. Eliav, E.; Ishikawa, Y. (1995). "Transition energies of ytterbium, lutetium, and lawrencium by the relativistic coupled-cluster method". Phys. Rev. A. 52: 291–296. Bibcode:1995PhRvA..52..291E. doi:10.1103/PhysRevA.52.291.
3. Zou, Yu; Froese, Fischer C. (2002). "Resonance Transition Energies and Oscillator Strengths in Lutetium and Lawrencium". Phys. Rev. Lett. 88 (18): 183001. Bibcode:2002PhRvL..88b3001M. doi:10.1103/PhysRevLett.88.023001. PMID 12005680.
4. Barbalace, Kenneth. "Periodic Table of Elements". Environmental Chemistry.com. Retrieved 2007-04-14.
5. "WebElements Periodic Table of the Elements". Webelements.com. Retrieved 2010-04-03.
6. "Periodic Table of the Elements". International Union of Pure and Applied Chemistry. Retrieved 2010-04-03.
7. ^ "Visual Elements". Royal Society of Chemistry. Retrieved 4 July 2011.
8. Dolg, Michael. "Lanthanides and Actinides" (PDF). Max-Planck-Institut für Physik komplexer Systeme, Dresden, Germany. CLA01. Retrieved 4 July 2011.

External links

• Dead-end pages from April 2014
• Periodic table