Which of the following factors may be regarded as the main...

Which of the following factors may be regarded as the main cause of lanthanide contraction? (a) Poor shielding of one of $4 \mathrm{f}$ electrons by another in the subshell (b) Effective shielding of one of $4 \mathrm{f}$ electrons by another in the subshell (c) Poor shielding of 5 d-electrons by 4 f electrons (d) Greater shielding of 5 d-electrons by $4 \mathrm{f}$ electrons

Which of the following factors may be regarded as the main cause of lanthanide contraction?
(a) Poor shielding of one of $4 \mathrm{f}$ electrons by another in the subshell
(b) Effective shielding of one of $4 \mathrm{f}$ electrons by another in the subshell
(c) Poor shielding of 5 d-electrons by 4 f electrons
(d) Greater shielding of 5 d-electrons by $4 \mathrm{f}$ electrons

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Key Concepts

Electron Shielding

Electron shielding is the effect whereby inner-shell electrons reduce the effective nuclear charge experienced by outer-shell electrons. In the context of the lanthanide contraction, the 4f electrons are poor at shielding the outer electrons from the nuclear charge, leading to a higher effective nuclear charge and a consequent contraction in the size of the atom or ion.

Effective Nuclear Charge

Effective nuclear charge is the net positive charge experienced by an electron in an atom, taking into account both the actual nuclear charge and the shielding effect of inner electrons. In the lanthanide series, because the 4f electrons shield poorly, the effective nuclear charge increases more than expected as protons are added, contributing to the phenomenon of lanthanide contraction.

Lanthanide Contraction

Lanthanide contraction refers to the progressive decrease in the atomic and ionic radii of the lanthanide elements with increasing atomic number. This contraction arises because the additional electrons fill the 4f subshell, which does not effectively shield the increasing nuclear charge, resulting in a stronger pull on the outer electrons and thus a reduction in size.

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