In a poly-electronic atom, the effective nuclear charge (Zeff) felt by the outermost electron must be larger than the nuclear charge (Z).

Question

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The prompt asks us to consider the statement about effective nuclear charge in a poly-electronic atom and compare Zeff to Z.
First, restating the central idea: in multi-electron atoms, electrons experience a net nuclear attraction that is reduced by shielding from other electrons, so the outermost electrons feel a Zeff that is not simply equal to Z.
Option: True. The assertion here is that Zeff felt by the outermost electron must be larger than Z. This is inconsistent with basic atomic structure: inner electrons shield the outer electrons, reducing the effective pull of the nucleus. Therefore Zeff is typically less than Z, not greater.
Option: False. This is the statement that aligns with established shielding concepts. In a poly-electronic atom, inner electrons screen some of the nuclear charge, so Zeff = Z − S, where S is the shielding constant due to other electrons. As a result, Zeff is always smaller than Z for the outermost electron, not larger. The degree of shielding can vary with electron configuration, but the fundamental inequality Zeff < Z remains generally true for multi-electron atoms.
Additional nuance: while Zeff can differ for electrons in different shells and subshells due to varying shielding and penetration, none of these factors typically produce Zeff > Z for any electron in a standard poly-electronic atom. The concept of shielding also explains trends in ionization energy and atomic radius across the periodic table. In contrast to the false statement, some might wonder about extreme cases or relativistic effects, but for ordinary chemistry contexts, Zeff being less than Z is the accepted norm.
In summary, the core idea is that shielding by other electrons reduces the effective nuclear charge felt by outer electrons, making Zeff smaller than Z rather than larger. This underpins why the given statement is considered incorrect in standard poly-electronic atoms.

CHEM1012 (L2) Online Quiz #1

In a poly-electronic atom, the effective nuclear charge (Zeff) felt by the outermost electron must be larger than the nuclear charge (Z).

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