Actinoid contraction explained

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]} 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 [<nowiki/>[[Radon|Ra]]])	M^3+ electron configuration	M^3+ radius (pm) (6-coordinate)
Ac	6d^17s^2	5f^0	111
Th	6d^27s^2	5f^1
Pa	5f^26d^17s^2	5f^2
U	5f^36d^17s^2	5f^3	103
Np	5f^46d^17s^2	5f^4	101
Pu	5f^67s^2	5f^5	100
Am	5f^77s^2	5f^6	99
Cm	5f^76d^17s^2	5f^7	99
Bk	5f^97s^2	5f^8	98
Cf	5f^107s^2	5f^9	98
Es	5f^117s^2	5f^10
Fm	5f^127s^2	5f^11
Md	5f^137s^2	5f^12
No	5f^147s^2	5f^13
Lr	5f^146d^17s^2	5f^14

Notes and 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 . en . 117 . 24 . 6597–6598 . 10.1021/ja00129a026 . 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 . 10.1063/1.477686 . 1998JChPh.10910806L . 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 . 6 January 2024.
4. Jolly, William L. Modern Inorganic Chemistry, McGraw-Hill 1984, p. 22