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Write a short note on lanthanoid contraction.

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Lanthanoid contraction:

i. The atomic and ionic radii of lanthanoids show gradual decrease with increase in atomic number. It is known as Lanthanoid contraction. 

ii. There are 14 Lanthanoids from Ce to Lu. In Lanthanoids, after Lanthanum (La), the electrons are added to prepenultimate shell i.e., 4f-orbital. 

iii. For each electron, one proton is also added to the nucleus of the atom of the element. Hence from Ce to Lu as atomic number increases, nuclear charge increases, therefore nuclear attraction increases. As atomic number increases, atomic volume or radius decreases as observed with all the elements along the period. 

iv. But in case of Lanthanoids, this decrease in atomic volume or radius is comparatively very small. This is explained in terms of Lanthanoid contraction.

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