A yellow metallic powder is buent in a stream of fluorine to...

A yellow metallic powder is buent in a stream of fluorine to obtain a colourless gas $(A)$ which is thermally stable and chemically inert. Its molecule has octahedral geometry. Anocher colourless gas (B) with same constituenc atoms as that of $(\Lambda)$ is obtained when sulphur dichloride is heated with sodium fluoride. Its molecule has trigonal bipyramidal geomecry, Gases $(\Lambda)$ and (B) are, respectively (a) $\mathrm{SF}_{6}$ and $\mathrm{SF}_{4}$ (b) $\mathrm{SF}_{4}$ and $\mathrm{SF}_{6}$ (c) $\mathrm{SF}_{4}$ and $\mathrm{S}_{2} \mathrm{~F}_{2}$ (d) $\mathrm{KCl}$ and $\mathrm{KBr}$

A yellow metallic powder is buent in a stream of fluorine to obtain a colourless gas $(A)$ which is thermally stable and chemically inert. Its molecule has octahedral geometry. Anocher colourless gas (B) with same constituenc atoms as that of $(\Lambda)$ is obtained when sulphur dichloride is heated with sodium fluoride. Its molecule has trigonal bipyramidal geomecry, Gases $(\Lambda)$ and
(B) are, respectively
(a) $\mathrm{SF}_{6}$ and $\mathrm{SF}_{4}$
(b) $\mathrm{SF}_{4}$ and $\mathrm{SF}_{6}$
(c) $\mathrm{SF}_{4}$ and $\mathrm{S}_{2} \mathrm{~F}_{2}$
(d) $\mathrm{KCl}$ and $\mathrm{KBr}$

Key Concepts

Octahedral Geometry

Octahedral geometry describes a molecular structure in which a central atom is symmetrically surrounded by six atoms or groups at the vertices of an octahedron. This arrangement minimizes electron?pair repulsions under the VSEPR model and is seen in compounds with six bonding pairs and no lone pairs. A well-known example involves a sulfur atom bonded to six fluorine atoms forming SF6, which is characteristically very stable due to this symmetrical, low-repulsion arrangement.

Trigonal Bipyramidal Geometry

Trigonal bipyramidal geometry is observed in molecules with five regions of electron density around a central atom. Under the VSEPR framework, these five pairs arrange themselves to minimize repulsions, forming a structure with three atoms in an equatorial plane and two in axial positions. In cases where there is a lone pair among the five regions, the resulting molecular shape may deform (e.g., to a seesaw shape), yet the electronic arrangement remains trigonal bipyramidal. This concept is central for understanding compounds with five electron groups, such as in derivatives formed during halogen exchange reactions.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a fundamental model in chemistry used to predict the geometry of molecules. It is based on the idea that electron pairs around a central atom will position themselves as far apart as possible to minimize repulsive forces. This theory helps chemists understand why molecules adopt specific shapes—such as octahedral for six bonding pairs or trigonal bipyramidal for five electron domains—and is key in relating a molecule’s structure to its reactivity and stability.

Halogenation and Fluorination Reactions

Fluorination reactions involve the addition or substitution of fluorine atoms, often using reactive fluorine gas, to form new compounds. Due to fluorine's high electronegativity and reactivity, these reactions can yield compounds that are thermally stable and chemically inert. Such reactions, including exchanges where one halogen is replaced by fluorine, help in synthesizing highly symmetrical and inert molecules, providing important insights into the structure and properties of halide compounds.

Thermal Stability and Chemical Inertness

Thermal stability and chemical inertness refer to a compound's ability to resist decomposition or reaction under a wide range of temperatures and conditions. Molecules such as those with a highly symmetrical octahedral structure (e.g., SF6) benefit from minimal electron-pair repulsions and evenly distributed charge, factors that contribute to their durability and lack of reactivity. Recognizing these properties is crucial when predicting the behavior of compounds in various chemical environments.

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