Lanthanide contraction As move along the lanthanide series from lanthanum to lutiteum, there is continuous decrease in the size of lanthanide ion with increase in atomic number. This decrease is known as lanthanide contraction. ...See More Photo: Lanthanide contraction As move along the lanthanide series from lanthanum to lutiteum, there is continuous decrease in the size of lanthanide ion with increase in atomic number. This decrease is known as lanthanide contraction. Lanthanide contraction may be defined as the steady decrease in size of lanthanide ions with increase in atomic number. Thus in the lanthanide series, lanthanum has the largest radius and lutetium has the smallest radius. Cause of Lanthanide contraction The cause of lanthanide contraction is generally attributed imperfect shielding of one 4f electron by another in the same shell. Thus as move along the Lanthanide series, the nuclear charge and the number of 4f electrons increase by one unit each step. However due to imperfect shielding because of the shape of f orbitals is very much diffused the effective nuclear charge increases which cause contraction in the size of the electron charge cloud and thus each ion shrinks in comparison with its predecessor. For example, in going from La3+ to Lu3+, the ionic radii decreases from 106 pm to 85pm. Consequences of Lanthanide contraction, 1. Electronegativity – there is slight increase in electronegativity of trivalent ions. 2. Ionic radius – there is steady decrease in the ionic size 3. Basicity difference – there is decrease in basic strength of the oxides and hydroxides of lanthanides with increase in atomic number. Thus La(OH)3 is the most basic while Lu(OH)3 is the less basic. Due to lanthanide contraction, the decrease in the size of the cation increase s the covalent character between the lanthanide ion and the hydroxide ion (according to Fajan’s rules) thereby increasing the basic character of the lanthanide hydroxisides. 4. E ° values – there is a small but smooth increase in standard electrode potential values, E ° for M3+ + 3e ------ > M(g) Lanthanum = -2.52V Lutetium = - 2.25V
Posted on: Sat, 27 Jul 2013 12:04:00 +0000
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