Actinide contraction

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.^[1] 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.^[2]

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.^[3]^[4]^[5]^[6] 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.

Element	Atomic electron configuration (all begin with [Ra])	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

References

^ Seth, Michael; Dolg, Michael; Fulde, Peter; Schwerdtfeger, Peter (June 1995). "Lanthanide and Actinide Contractions: Relativistic and Shell Structure Effects". Journal of the American Chemical Society. 117 (24): 6597–6598. doi:10.1021/ja00129a026. ISSN 0002-7863.
^ Laerdahl, J. K.; Fægri, K.; Visscher, L.; Saue, T. (1998-12-22). "A fully relativistic Dirac–Hartree–Fock and second-order Mo/ller–Plesset study of the lanthanide and actinide contraction". The Journal of Chemical Physics. 109 (24): 10806–10817. Bibcode:1998JChPh.10910806L. doi:10.1063/1.477686. ISSN 0021-9606.
^ Chistyakov, V. M. (1968). "Biron's Secondary Periodicity of the Side d-subgroups of Mendeleev's Short Table". Journal of General Chemistry of the USSR. 38 (2): 213–214. Retrieved 6 January 2024.
^ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 536, 649, 743. ISBN 978-0-13-039913-7.
^ Cotton, F. Albert; Wilkinson, Geoffrey (1988), Advanced Inorganic Chemistry (5th ed.), New York: Wiley-Interscience, pp. 776, 955, ISBN 0-471-84997-9
^ Jolly, William L. Modern Inorganic Chemistry, McGraw-Hill 1984, p. 22

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[1] Seth, Michael; Dolg, Michael; Fulde, Peter; Schwerdtfeger, Peter (June 1995). "Lanthanide and Actinide Contractions: Relativistic and Shell Structure Effects". Journal of the American Chemical Society. 117 (24): 6597–6598. doi:10.1021/ja00129a026. ISSN 0002-7863.

[2] Laerdahl, J. K.; Fægri, K.; Visscher, L.; Saue, T. (1998-12-22). "A fully relativistic Dirac–Hartree–Fock and second-order Mo/ller–Plesset study of the lanthanide and actinide contraction". The Journal of Chemical Physics. 109 (24): 10806–10817. Bibcode:1998JChPh.10910806L. doi:10.1063/1.477686. ISSN 0021-9606.

[Chistyakov-3] Chistyakov, V. M. (1968). "Biron's Secondary Periodicity of the Side d-subgroups of Mendeleev's Short Table". Journal of General Chemistry of the USSR. 38 (2): 213–214. Retrieved 6 January 2024.

[Housecroft-4] Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 536, 649, 743. ISBN 978-0-13-039913-7.

[Cotton-5] Cotton, F. Albert; Wilkinson, Geoffrey (1988), Advanced Inorganic Chemistry (5th ed.), New York: Wiley-Interscience, pp. 776, 955, ISBN 0-471-84997-9

[Jolly-6] Jolly, William L. Modern Inorganic Chemistry, McGraw-Hill 1984, p. 22

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