Actinoid contraction

The actinoid contraction or actinide contraction is the greater-than-expected decrease in atomic radii and ionic radii of the elements in the actinoid series, from left to right.

Description

It is more pronounced than the lanthanoid 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 actinoid 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-actinoid 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

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.
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.
4. Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 536, 649, 743. ISBN 978-0-13-039913-7.
5. Cotton, F. Albert; Wilkinson, Geoffrey (1988), Advanced Inorganic Chemistry (5th ed.), New York: Wiley-Interscience, pp. 776, 955, ISBN 0-471-84997-9
6. Jolly, William L. Modern Inorganic Chemistry, McGraw-Hill 1984, p. 22