Question

Screening effect of inner shells decreases the effective nuclear charge. The order of screening effect of subshells is s > p > d > f. With respect to the above facts, predict the gradation of atomic size in lanthanide elements.

Answer 1

Hint: The Shielding effect is when the electron and the nucleus in an atom have a decrease in attraction which changes the nuclear charge. Example, in nuclear fission when electrons farthest from the center of the atom are pulled away.
Now, apply the concept for predicting the gradation.

Complete step by step answer:

The Lanthanide Contraction is the result of 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 affected by nuclear charge. So when the shielding is not as good, this would mean the positively charged nucleus has a greater attraction to the electrons, thus decreasing the atomic radius as the atomic number increases. The Lanthanide Contraction can be seen by comparing the elements with f electrons and those without F electrons in d block orbital.The decrease in the force of attraction exerted by the nucleus on the valence electrons due to the presence of electrons in the inner shells is called screening effect or shielding effect.
Let us see what the shielding effect is:
In a multi-electron atom the valence electron experiences attraction by the nucleus and repulsion by the electrons of inner-shells. This combined effect of attraction and repulsion force acting on the valence-electron, makes the electron experience less attraction from the nucleus. This is called Shielding Effect.
Therefore, s has the greatest effect to reduce the force of attraction between the outermost electrons and nucleus due to its effective charge density followed by s > p > d > f.

Note: The Lanthanide metals are silvery white but the trivalent lanthanide ions show different colors. Consequences of Lanthanide Contraction are:

1. Atomic size
2. Difficulty in the separation of lanthanides.
3. Complex formation
4. The ionization of energy of d-block elements.