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

The actinide contraction is more pronounced than the lanthanide contraction since 5f electrons are less effective at shielding than 4f electrons. 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.

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. This effect causes the radii of transition metals of groups 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.

See also

• Lanthanide contraction