In each of the following pairs of substances, one is stable...

In each of the following pairs of substances, one is stable and known, and the other is unstable. For each pair, choose the stable substance, and explain why the other is unstable. a. $\mathrm{NF}_{5}$ or $\mathrm{PF}_{5} \quad$ b. AsF_r $\mathrm{AsI}_{5} \quad$ c. $\mathrm{NF}_{3}$ or $\mathrm{NBr}_{3}$

In each of the following pairs of substances, one is stable and known, and the other is unstable. For each pair, choose the stable substance, and explain why the other is unstable.
a. $\mathrm{NF}_{5}$ or $\mathrm{PF}_{5} \quad$ b. AsF_r $\mathrm{AsI}_{5} \quad$ c. $\mathrm{NF}_{3}$ or $\mathrm{NBr}_{3}$

Key Concepts

Octet Rule and Expanded Valence Shell

The octet rule governs how atoms achieve stability by having eight electrons in their valence shell. However, atoms from the third period and beyond (like phosphorus and arsenic) can have an expanded valence shell because they have accessible d orbitals. In contrast, second?period elements such as nitrogen cannot expand their valence shell beyond eight electrons, making molecules like NF5 unstable compared to their analogues (PF5).

Atomic Size and Steric Factors

The size of the central atom and the substituent groups plays a crucial role in molecular stability. Larger atoms can more easily accommodate several bonds without excessive electron–electron repulsion. Also, when the substituents themselves are large, as in the case when iodine or bromine are involved, steric hindrance can destabilize the molecule. This is why compounds like AsI5 or NBr3 are less stable relative to their counterparts with smaller halogens.

Electronegativity and Bond Strength

Electronegativity affects the polarity and strength of chemical bonds. Highly electronegative atoms form strong bonds due to effective electron sharing and stabilization of charge distribution. For instance, fluorine, being very electronegative, when bonded to elements that can suit the required geometry and electron count (like phosphorus or arsenic), leads to stable compounds. Conversely, when less electronegative atoms such as bromine or iodine are involved, the bonds formed are weaker and can render the molecule unstable.

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