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What is lanthanoid contraction? Mention one of its consequences.

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Hint:
Victor Goldschmidt gave the term Lanthanoid contraction in 1925. There are a total of 15 elements in the lanthanide series. It shows some peculiar properties such as a decrease in the ionic radius.

Complete step by step answer:
-Lanthanoid contraction is the abnormal decrease in the ionic and atomic radii with increasing atomic number.
-The elements from 57 to 71 atoms number i.e. from lanthanum to lutetium comes under the lanthanoid contraction.
-The lanthanide have the general configuration of $\text{5}{{\text{f}}^{1-14}}6{{\text{d}}^{1}}\text{7}{{\text{s}}^{2}}$.
-The cause of the lanthanoid contraction is the poor shielding effect of 4f orbital due to which the electrons of s-orbitals attract more towards the nucleus due to which the size of the atoms decreases.
-Lanthanoids are divalent in metallic form.
-The element of lanthanoid series which is considered as the hardest substance is lutetium and also has a high melting point.
CONSEQUENCES:
-Due to the decrease in the atomic radii, the ionisation energy as well as electronegativity increases. Because the atomic radius is inversely proportional to the ionisation energy and electronegativity.
-They are hard to separate because of the strong attachment of the outermost electron with the nucleus it is hard to separate the lanthanides.

Note:
Due to the decrease in the atomic radii of the lanthanides, it is observed that the basicity of the elements also decreases because of the increase in the covalent bond (according to the Fajan's rule). Actinides are the elements with the atomic number from 89 to 103.