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What is lanthanide contraction ? Give its cause. What are the consequences of lanthanide contraction ? |
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Answer» Solution :In the lanthanides, there is a regular decrease in the size of atoms and tripositive ions with increase in atomic number. This decrease of ionic and atomic RADII in the lanthanide elements is called lanthanide contraction. For example, the ionic radii decrease from `Ce^(3+) (1.11 Å)` to `Lu^(3+) (0.93 Å)`. Cause of lanthanide contraction. As we move from LEFT to right in the lanthanides, for EVERY proton added to the nucleus, the additional electron enters 4f orbital. But 4f electrons are too diffused to effectively screen the nucleus. Due to this poor or ineffective shielding by 4f electrons, the effective nuclear charge increases and size decreases. Consequences of lanthanide contraction. The important consequence of lanthanide contration are (i) Chemical similarilty. Due to lanthanide contraction, the increase in radii from SECOND to third transition series vanishes. Consequently, the pairs of elements Zr -- Hf, Nb – Ta, Mo – W, etc. have almost similar sizes and therefore, the properties of these elements are similar. (ii) There is small but STEADY increase in the standard electrode potential values `(E^(@)) (M^(3+) + 3e^(-) hArr M)`for these elements, (iii) The basic strength of oxides and hydroxides decrease with increase in atomic number. Thus, `La (OH)_(3)` is most basic and`Lu (OH)_(3)` is the least basic. |
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