Question
Explain the lanthanoid contraction.
✓
Answer
$\rightarrow$ As we move along the lanthanoid series, the atomic number increases gradually by one. This means that the number of electrons and protons present in an atom also increases by one.
$\rightarrow$ As electrons are being added to the same shell, the effective nuclear charge increases.
$\rightarrow$ This happens because the increase in nuclear attraction due to the addition of proton is more pronounced than the increase in the interelectronic repulsions due to the addition of electron.
$\rightarrow$ Also, with the increase in atomic number, the number of electrons in the $4f$ orbital also increases. The $4f$ electrons have poor shielding effect.
$\rightarrow$ Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the attraction of the nucleus for the outermost electrons increases.
$\rightarrow$ This results in a steady decrease in the size of lanthanoids with the increase in the atomic number. This is termed as lanthanoid contraction.
$\rightarrow$ Consequences of lanthanoid contraction
$\rightarrow$ There is similarity in the properties of second and third transition series.
$\rightarrow$ Separation of lanthanoids is possible due to lanthanoid contraction.
$\rightarrow$ It is due to lanthanoid contraction that there is variation in the basic strength of lanthanoid hydroxides. $($Basic strength decreases from $\ce{La(OH)_3}$ to $ \ce{Lu(OH)_3}.)$
$\rightarrow$ As electrons are being added to the same shell, the effective nuclear charge increases.
$\rightarrow$ This happens because the increase in nuclear attraction due to the addition of proton is more pronounced than the increase in the interelectronic repulsions due to the addition of electron.
$\rightarrow$ Also, with the increase in atomic number, the number of electrons in the $4f$ orbital also increases. The $4f$ electrons have poor shielding effect.
$\rightarrow$ Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the attraction of the nucleus for the outermost electrons increases.
$\rightarrow$ This results in a steady decrease in the size of lanthanoids with the increase in the atomic number. This is termed as lanthanoid contraction.
$\rightarrow$ Consequences of lanthanoid contraction
$\rightarrow$ There is similarity in the properties of second and third transition series.
$\rightarrow$ Separation of lanthanoids is possible due to lanthanoid contraction.
$\rightarrow$ It is due to lanthanoid contraction that there is variation in the basic strength of lanthanoid hydroxides. $($Basic strength decreases from $\ce{La(OH)_3}$ to $ \ce{Lu(OH)_3}.)$
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