For each of the following pairs, indicate which species is...

For each of the following pairs, indicate which species is smaller. Explain your reasoning in terms of the electron structure of each species.
a. H or $\mathrm{H}^{-}$
b. $\mathrm{N}$ or $\mathrm{N}^{3-}$
c. Al or $\mathrm{Al}^{3+}$
d. F or Cl

Key Concepts

Effective Nuclear Charge

Effective nuclear charge is the net positive charge experienced by an electron in an atom or ion after accounting for the repulsive effects of other electrons. A stronger effective nuclear charge pulls electrons closer to the nucleus, resulting in a smaller radius, while a lower effective nuclear charge allows the electron cloud to expand.

Electron-Electron Repulsion

Electron-electron repulsion arises from the mutual repulsion between negatively charged electrons. This repulsion counteracts the nuclear attraction and can cause the electron cloud to spread out, thereby increasing the size of the atom or ion, especially when additional electrons are present.

Atomic and Ionic Radii

Atomic and ionic radii refer to the sizes of neutral atoms and ions, respectively. In general, cations (which have lost electrons) are smaller than their neutral counterparts due to a reduced electron-electron repulsion and a relatively higher effective nuclear charge, while anions (which have gained electrons) are larger because of increased electron-electron repulsion and shielding effects.

Isoelectronic Series

An isoelectronic series consists of species that have the same number of electrons but differ in their nuclear charges. Within an isoelectronic series, the size of the species decreases as the nuclear charge increases because a higher nuclear charge more strongly attracts the same number of electrons, leading to a smaller radius.

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