The lanthanide contraction is due to:a)Poor shielding effect of 4fb)Filling of 4d before 4fc)Poor shielding effect 5dd)Filling of 4f before 4dCorrect answer is option 'A'. Can you explain this answer?

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Answer

The Lanthanide Contraction

The lanthanide contraction is a phenomenon observed in the elements of the lanthanide series, where there is a decrease in the atomic and ionic radii of the elements as one moves from left to right across the series. This contraction is mostly due to the poor shielding effect of 4f electrons.

Poor Shielding Effect of 4f Electrons

The sub-shell 4f is characterized by its low level of effective nuclear charge and poor shielding effect. This means that the 4f electrons do not effectively shield the outermost electrons from the nuclear charge, resulting in an increase in effective nuclear charge across the series. As a result, the electrons are pulled in closer to the nucleus, leading to a decrease in the atomic and ionic radii.

Filling of 4f Before 5d

Another factor that contributes to the lanthanide contraction is the filling of the 4f sub-shell before the 5d sub-shell. This means that the 4f electrons are filled before the 5d electrons, leading to an increase in the effective nuclear charge experienced by the 5d electrons. This also contributes to the decrease in atomic and ionic radii across the series.

Conclusion

In conclusion, the lanthanide contraction is mostly due to the poor shielding effect of the 4f electrons, which results in an increase in effective nuclear charge across the series. The filling of the 4f sub-shell before the 5d sub-shell also contributes to the contraction.

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