The physical and chemical properties of the 2nd and 3rd transition series are almost the same as that of the first transition series.Why?

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On moving down the group, size of atom increases because of extra shells of electrons present. The elements of first group shows expected increase in size. However, in subsequent groups there is an increase in radius 1-2 pm between the first and second transition series,  but hardly any difference between the second and third transition series. This is due to lanthanide contraction. In the lanthanide series, 4f shell of electrons is filled which is a poor shielder due to the diffused shape of the f-subshell which leads to gradual decrease in the size of atom. The lanthanide contraction cancels almost exactly the normal size increase due to addition of more shells. Therefore, size of elements of second and third transition series are almost same and so are their chemical and physical properties. 

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Size did not increase which it has to be increase due to lanth. Contraction
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