Covalent and ionic radii normally increase on descending a group in the periodic table due to the presence of extra filled shells of electrons. On moving from left to right across a period, the covalent and ionic radii decrease. This is because the extra orbital electrons incompletely shield the extra nuclear charge. Thus, all the electrons are pulled in closer. The shielding effect of electrons decreases in the order s > p > d > f. The contraction in size from one element to another is fairly small. However, the additive effect over the 14 lanthanide elements from Ce to Lu is about 0.2 Angstrom, and this is known as the lanthanide contraction.
The physical properties such as hardness, melting points and boiling points of the elements all increase from Ce to Lu. This is because the attraction between the atoms increases as the size decreases.
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The properties of an ion depend on its size and its charge. The Ln3+ lanthanide ions change by only a small amount from one element to the next, and their charge is the same, and so their chemical properties are very similar. Since Lu3+ is the smallest ion, it is the most heavily hydrated. Though the lanthanides do not form complexes very extensively since Lu3+ is the smallest ion the complexes formed by Lu3+ are the strongest. La3+ and Ce3+ are the largest ions so La(OH)3, and Ce(OH)3 are the strongest bases.
Effects Of Lanthanide Contraction
The lanthanide contraction reduces the radii of the last four elements in the series below that for Y in the preceding transition series. Since the size of the heavier lanthanide ions, particularly Dy3+ and Ho3+, are similar to that of Y3+ it follows that their chemical properties are also very similar. As a result, the separation of these elements is very difficult.
Ionic radii depend on the number of electrons removed. For simplicity, the radii of ions with the same charge are compared in the Table. A similar change in size across the series is observed if covalent radii are compared.
Because of this contraction in size across the lanthanide series, the elements which follow in the third transition series are considerably smaller than would otherwise be expected. The normal size increase Sc → Y → La disappears after the lanthanides. Thus pairs of elements such as Zr/Hf, Nb/Ta and Mo/W are almost identical in size. The close similarity of properties in such a pair makes chemical separation very difficult. The sizes of the third-row transition elements are very similar to those of the second-row transition elements. Thus the second and third rows of transition elements resemble each other more closely than do the first and second rows.
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