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==