The rare earth elements of the modern periodic table are known as lanthanides. They have a separate block in the modern periodic table. The lanthanide series consists of fourteen elements starting from cerium (atomic number 58) to lutetium (atomic number 71) in the periodic table. These elements strongly resemble lanthanum (atomic number 57) and hence the series is called lanthanide series. The valence shell of lanthanides remains unchanged, yet there is a decrease in the atomic size and ionic size with increase in atomic number due to lanthanide contraction. Lanthanide contraction is a very important phenomenon in determining the characteristics of the elements of lanthanide series.
Consequences of Lanthanide Contraction
Following points will clearly depict the effect of lanthanide contraction:
1. Atomic and ionic size: In general the atomic size of a group increases with an increase in atomic number. This increase in size is due to the addition of a new shell. But this trend is not followed by all the groups of the transition series due to lanthanide contraction. The effect of lanthanide contraction cancels the expected increase in atomic size.
2. Basicity of oxides and hydroxides: With an increase in the atomic number, the basic strength of the oxides and hydroxides decreases. This contraction causes a decrease in the size of lanthanide cations and, therefore, the polarizing power of the cations increases. This further decreases the ionic character of the oxides and hydroxides.
3. Density: A reduction in the atomic and ionic size of the element causes an increase in the value of density. In group 3, density increases very regularly as we move form Sc to La, but in group 4 and the subsequent groups the value of density is almost double to those of the corresponding second row elements. So it can be concluded that due to lanthanide contraction there is an abnormal increase in the density of these elements
4. Ionization potential: The values of ionization potential should be lower and should decrease regularly down the group. But due to lanthanide contraction there is no trend seen in the values of ionization potential, this irregularity occurs after the element tungsten.
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