Lanthanide contraction is defined as a gradual decrease in the atomic and ionic radii (M3+ ions) of lanthanide elements as the atomic number increases. Covalent and ionic radii, due to the presence of extra-filled electrons, generally increase on descending a group in the periodic table. However, on increasing Z i.e. moving from left to right across the lanthanide series, the atomic and ionic radii (M3+ ions) rapidly decreases as the extra orbital electrons incompletely shield the extranuclear charge causing all the electrons to be drawn closer.
Cause of Lanthanide Contraction
The Lanthanide contraction is caused by the poor shielding effects of the extra 4f orbital electrons and a gradual increase in the nuclear charge. The inclusion of extra electrons in the 4f orbital and the addition of extranuclear charge is known to occur as we progress through the lanthanide series from one element to the next.
The extra orbital electrons (4f electrons) in completely shield the extranuclear charge. Since shielding efficiency is in the order s>p>d>f, f electrons are least effective in shielding the nuclear charge. Thus, inner electrons’ decreased shielding of the nuclear charge causes significant attraction between the nucleus and outer electrons, leading the nucleus to contract in size.
Consequences of Lanthanide Contraction
Some of the consequences of lanthanide contraction are:
- The size and charge of an ion determine the properties of Lanthanides. The Ln3+ ions have a common charge (+3), and the diameters of two neighboring Ln3+ ions do not differ significantly, even though there is an overall contraction from La3+ to Lu3+. As a result, the chemical properties of a lanthanide ion and its next ion are nearly similar. Because of their chemical similarity, it is quite difficult in separating lanthanides.
- Because of lanthanide contraction, the radius of Dy3+ (0.912 Å) and Ho3+ (0.901 Å) is nearly identical to that of Y3+ (0.900 Å), which is above them in the preceding transition series. Thus, compounds of these three ions have identical properties due to similarities in their size.
- The elements in the third transition series are significantly smaller than would be predicted due to the contraction in size over the lanthanide series. The normal size increases from Sc to Y to La and disappears after the lanthanides. As a result, the sizes of elements like Zr/Hf, Nb/Ta, and Mo/W are nearly similar. The transition elements in the third row are quite similar in size to those in the second row. As a result, the transition elements in the second and third rows are more similar to those in the first and second rows.
- Since the size of +3 lanthanide ions (Ln3+) decreases with an increase in atomic number due to lanthanide contraction, the covalent character between the Ln3+ ion and the OH- ion increases from La(OH)3 to Lu(OH)3. According to Fajan’s rule, smaller positive ions can cause greater polarization of the anion. As a result, as the atomic number increases, the hydroxide’s basic character decreases.
Lanthanide Contraction Video
FAQs/MCQs
Define lanthanide contraction
The steady decrease in atomic and ionic radii (M3+ ions) of lanthanide elements with increasing atomic number is called lanthanide contraction.
4f block elements
4f elements are called lanthanides since the element in this block is chemically identical to Lanthanum.
References
- J. D. Lee, Concise Inorganic Chemistry, 5th Edition, John Wiley and Sons. Inc. 2007.
- F. A. Cotton, G. Wilkinson & C. Gaus, Basic Inorganic Chemistry, 3 rd Edition, John Wiley & Sons (Asia), Pvt., Ltd., 2007.
- D. F. Shriver & P. W. Atkins, Inorganic Chemistry, 5th Edition, Oxford University Press, 2010.
