Which member of the following pairs would you expect to be...

Which member of the following pairs would you expect to be more energetically stable? Justify each choice.
a. $\mathrm{NaBr}$ or $\mathrm{NaBr}_{2}$
b. $\mathrm{ClO}_{4}$ or $\mathrm{ClO}_{4}$
c. $\mathrm{SO}_{4}$ or $\mathrm{XeO}_{4}$
d. $\mathrm{OF}_{4}$ or $\mathrm{SeF}_{4}$

Key Concepts

Atomic Size and Structural Considerations

The size of the central atom plays a crucial role in determining the overall stability of a compound. Larger atoms provide more space for bonding and can better accommodate multiple substituents, which may reduce steric strain and unfavorable repulsions. Differences in atomic size can therefore lead to significant variations in the energetic stability of otherwise similar molecular formulas.

Expanded Octets and Hypervalency

Some elements, notably those in the third period or below, can exceed the octet rule by utilizing d-orbitals, forming hypervalent compounds. The effectiveness of this expanded octet in stabilizing a molecule depends on the central atom's ability to distribute electron density over several bonds. This concept explains why species centered on elements that readily expand their octet can be either more or less stable depending on the overall electronic and steric factors at play.

Oxidation State and Electronegativity

The stability of a compound depends significantly on the oxidation state of the central atom and the relative electronegativity of the atoms involved. Atoms with lower electronegativity can generally stabilize higher oxidation states, while highly electronegative atoms may struggle to support an unusually high positive or negative charge, leading to less stable compounds.

Ionic Bonding and Lattice Energy

In ionic compounds, the overall energetic stability is largely determined by the lattice energy, which is the energy released when oppositely charged ions pack into a regular, solid structure. A compound that forms a well-organized ionic lattice tends to be more stable because strong electrostatic interactions lower its energy compared to atypical ionic species with less optimized charge distributions.

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