Question

what is lanthanoid contraction?

Answer 1

the steady decreases in the size of the atoms and ions of the rare earth elementswith increasing atomic number from lanthanum through lelutium
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Answer 2

Hey !!

In lanthanoids, with increasing atomic number, the atomic and ionic radii decreases from one element to the other, but the decrease is very small. It is because, for every additional proton in the nucleus, the corresponding electron goes into a 4f-subshell, which is too diffused to screen the nucleus as effectively as the more localised inner shell. Hence, the attraction of the nucleus for the outermost electrons increases steadily with the atomic increases.