Question
Thu March 03, 2011 By: Mohit Gehlot
 

What is the cause of Lanthanoid Contraction? Also what do u undertand by the term"shieldinf of 4f electrons"?

Expert Reply
Thu March 03, 2011

Dear Student

 The atomic radius, as according to the Lanthanide Contraction, of these elements decreases as the atomic number increases.

The Lanthanide Contraction is the result of a poor shielding effect of the 4f electrons. The shielding effect is described as the phenomenon by which the inner-shell electrons shield the outer-shell electrons so they are not effected by nuclear charge. So when the shielding is not as good, this would mean that the positively charged nucleus has a greater attraction to the electrons, thus decreasing the atomic radius as the atomic number increases. The S orbital has the greatest shielding while F has the least and P and D in between the two with P being greater than D. 

We hope that clarifies your query.

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