Question

The 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

Screening effect:

• When the number of inner electrons is greater, they shield the outermost electron from the nucleus so that the outermost electron becomes free from any nuclear attraction.
• This is called the shielding or screening effect.
• In a multielectron atom, the valence shell’s electrons are attracted to the nucleus, and these electrons are repelled by the electrons present in the inner shells.
• On account of this, the actual force of attraction between the nucleus and the valence electrons is decreased by the repulsive forces acting in opposite directions.
• This 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 the screening effect or shielding effect.

Gradation of atomic size in lanthanide elements:

• The steady decrease in the atomic size of lanthanide elements due to the increase in atomic number is known as lanthanide contraction.
• Lanthanide contraction is a result of the poor screening effect of the 4f electrons. So the order of screening effect of subshells is s>p>d>f.
• 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. f orbitals do undergo shielding in lanthanide element series but it is a poor shielding effect.