Wiktionary
n. (context chemistry physics English) the progressive decrease in the radii of atoms of the lanthanide elements as the atomic number increases; evident in various physical properties of the elements and their compounds
Wikipedia
Lanthanide contraction is a term used in chemistry to describe the greater-than-expected decrease in ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to 71, lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, hafnium. The term was coined by the Norwegian geochemist Victor Goldschmidt in his series "Geochemische Verteilungsgesetze der Elemente".
Element
Ce
Atomic electron configuration
(all begin with [Xe])
5d6s
4f5d6s
4f6s
4f6s
4f6s
4f6s
4f6s
4f5d6s
4f6s
4f6s
4f6s
4f6s
4f6s
4f6s
4f5d6s
Ln electron configuration
4f
4f
4f
4f
4f
4f
4f
4f
4f
4f
4f
4f
4f
4f
4f
Ln radius (pm) (6-coordinate)
103
102
99
98.3
97
95.8
94.7
93.8
92.3
91.2
90.1
89
88
86.8
86.1