Lanthanide contraction

The lanthanide contraction is the steady decrease in the ionic radii of the elements in the lanthanide series, from left to right. It is caused by the imperfect shielding of the increasing nuclear charge by the 4f electrons. About 10% of the lanthanide contraction has been attributed to relativistic effects.

The 4f electrons continue to shield imperfectly as the 5d subshell begins to be filled. This results in smaller than otherwise expected atomic radii and ionic radii for the subsequent d-block elements starting with 71, lutetium. 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-lanthanide elements.

The atomic radii of the lanthanides also decreases across the group, but not uniformly as is the case with the 3+ ions:

Element	Atomic electron configuration (all begin with [Xe])	Ln3+ electron configuration	Ln3+ radius (pm) (6-coordinate)
La	5d16s2	4f0	103
Ce	4f15d16s2	4f1	102
Pr	4f36s2	4f2	99
Nd	4f46s2	4f3	98.3
Pm	4f56s2	4f4	97
Sm	4f66s2	4f5	95.8
Eu	4f76s2	4f6	94.7
Gd	4f75d16s2	4f7	93.8
Tb	4f96s2	4f8	92.3
Dy	4f106s2	4f9	91.2
Ho	4f116s2	4f10	90.1
Er	4f126s2	4f11	89
Tm	4f136s2	4f12	88
Yb	4f146s2	4f13	86.8
Lu	4f145d16s2	4f14	86.1

The term was coined by the Norwegian geochemist Victor Goldschmidt in his series "Geochemische Verteilungsgesetze der Elemente" (Geochemical distribution laws of the elements).