actinide contraction
{{short description|Decrease of ionic radii across the actinide series}}
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The actinide contraction is the greater-than-expected decrease in atomic radii and ionic radii of the elements in the actinide series, from left to right.
Description
It is more pronounced than the lanthanide contraction because the 5f electrons are less effective at shielding than 4f electrons.{{Cite journal |last1=Seth |first1=Michael |last2=Dolg |first2=Michael |last3=Fulde |first3=Peter |last4=Schwerdtfeger |first4=Peter |date=June 1995 |title=Lanthanide and Actinide Contractions: Relativistic and Shell Structure Effects |url=https://pubs.acs.org/doi/abs/10.1021/ja00129a026 |journal=Journal of the American Chemical Society |language=en |volume=117 |issue=24 |pages=6597–6598 |doi=10.1021/ja00129a026 |issn=0002-7863|url-access=subscription }} It is caused by the poor shielding effect of nuclear charge by the 5f electrons along with the expected periodic trend of increasing electronegativity and nuclear charge on moving from left to right. About 40-50% of the actinide contraction has been attributed to relativistic effects.{{Cite journal |last1=Laerdahl |first1=J. K. |last2=Fægri |first2=K. |last3=Visscher |first3=L. |last4=Saue |first4=T. |date=1998-12-22 |title=A fully relativistic Dirac–Hartree–Fock and second-order Mo/ller–Plesset study of the lanthanide and actinide contraction |url=https://pubs.aip.org/aip/jcp/article-abstract/109/24/10806/476506/A-fully-relativistic-Dirac-Hartree-Fock-and-second?redirectedFrom=fulltext |journal=The Journal of Chemical Physics |volume=109 |issue=24 |pages=10806–10817 |doi=10.1063/1.477686 |bibcode=1998JChPh.10910806L |issn=0021-9606}}
A decrease in atomic radii can be observed across the 5f elements from atomic number 89, actinium, to 102, nobelium. This results in smaller than otherwise expected atomic radii and ionic radii for the subsequent d-block elements starting with 103, lawrencium.{{cite journal |last1=Chistyakov |first1=V. M. |date=1968 |title=Biron's Secondary Periodicity of the Side d-subgroups of Mendeleev's Short Table |url=https://archive.org/details/sim_russian-journal-of-general-chemistry_1968-02_38_2/page/212/mode/2up |journal=Journal of General Chemistry of the USSR |volume=38 |issue=2 |pages=213–214 |doi= |access-date=6 January 2024}}{{Housecroft2nd|pages=536, 649, 743}}{{Cotton&Wilkinson5th|pages=776, 955}} Jolly, William L. Modern Inorganic Chemistry, McGraw-Hill 1984, p. 22 This effect causes the radii of transition metals of group 5 and 6 to become unusually similar, as the expected increase in radius going down a period is nearly cancelled out by the f-block insertion, and has many other far ranging consequences in post-actinide elements.
The decrease in ionic radii (M3+) is much more uniform compared to decrease in atomic radii.
| class="wikitable" | |||
| Element | Atomic electron configuration (all begin with [ | M3+ electron configuration | M3+ radius (pm) (6-coordinate) |
|---|---|---|---|
| Ac | 6d17s2 | 5f0 | 111 |
| Th | 6d27s2 | 5f1 | |
| Pa | 5f26d17s2 | 5f2 | |
| U | 5f36d17s2 | 5f3 | 103 |
| Np | 5f46d17s2 | 5f4 | 101 |
| Pu | 5f67s2 | 5f5 | 100 |
| Am | 5f77s2 | 5f6 | 99 |
| Cm | 5f76d17s2 | 5f7 | 99 |
| Bk | 5f97s2 | 5f8 | 98 |
| Cf | 5f107s2 | 5f9 | 98 |
| Es | 5f117s2 | 5f10 | |
| Fm | 5f127s2 | 5f11 | |
| Md | 5f137s2 | 5f12 | |
| No | 5f147s2 | 5f13 | |
| Lr | 5f146d17s2 | 5f14 |