Question

In which of the following pairs, the two species are iso-structural? (a) \(\mathrm{SF}_{4}\) and \(\mathrm{XeF}_{4}\) (b) \(\mathrm{SO}_{3}^{2}\) and \(\mathrm{NO}_{3}^{-}\) (c) \(\mathrm{BF}_{3}\) and \(\mathrm{NF}\) (d) \(\mathrm{BrO}_{3}^{-}\)and \(\mathrm{XeO}_{2}\)

Short answer

None of the pairs are iso-structural.

Step-by-step solution

Define Iso-structural Species

Iso-structural species are molecules or ions that have the same shape and the same type of hybridization at the central atom. This means they have identical geometrical structures and similar electron pair arrangements around the central atoms.

Analyze Option (a): SF₄ and XeF₄

SF₄ has a seesaw shape due to its trigonal bipyramidal electron pair geometry (one lone pair), while XeF₄ has a square planar shape with two lone pairs on xenon. These shapes are not the same, so they are not iso-structural.

Analyze Option (b): SO₃²⁻ and NO₃⁻

SO₃²⁻ (sulfite ion) has a trigonal pyramidal shape due to the presence of a lone pair on sulfur. NO₃⁻ (nitrate ion) is planar with no lone pairs on nitrogen. Since their shapes differ, they are not iso-structural.

Analyze Option (c): BF₃ and NF

BF₃ is a planar trigonal shape because the boron is surrounded by three fluorine atoms and has no lone pairs, but NF is not a common simple molecule; likely it refers to NF₃. NF₃ has pyramidal geometry due to lone pair presence, making the shapes different from BF₃. Thus, BF₃ and NF and incorrectly implied NF₃ are not iso-structural.

Analyze Option (d): BrO₃⁻ and XeO₂

BrO₃⁻ is a trigonal pyramidal ion due to the presence of a lone pair on bromine, while XeO₂ is typically a linear molecule. Their structures are different as well, thus they are not iso-structural.

Conclusion from Analysis

From the analysis of all options, each pair has differing molecular geometries and therefore no pair consists of iso-structural species. Contrary to the listed options, none of these pairs demonstrates iso-structurality.

Key concepts

Molecular Geometry

In chemistry, molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Understanding molecular geometry is crucial as it influences physical and chemical properties such as polarity, reactivity, phase of matter, color, magnetism, biological activity, and much more. Essentially, the shape of a molecule is determined by the regions of electron density around the central atom such as bonds and lone pairs. Here are some common molecular geometries:

• Linear: A straight-line arrangement, e.g., CO₂.
• Trigonal Planar: Three atoms arranged at the corners of an equilateral triangle around the central atom, e.g., BF₃.
• Tetrahedral: Four atoms forming a structure similar to a tripod, e.g., CH₄.
• Trigonal Bipyramidal: Five atoms where three lie in a plane and two above and below the plane, e.g., PCl₅.
• Square Planar: Four surrounding atoms form a square around the central atom, e.g., XeF₄.

Recognizing these geometries can help in predicting molecular behavior and properties.

Hybridization

Hybridization is a concept in which atomic orbitals mix to form new hybrid orbitals. These hybrid orbitals have different energies and shapes compared to the original atomic orbitals, which results in molecules having specific geometries.
Different types of hybridization account for different shapes:

• sp Hybridization: Linear shape with a bond angle of 180°.
• sp² Hybridization: Trigonal planar shape with bond angles of 120°.
• sp³ Hybridization: Tetrahedral shape with bond angles of 109.5°.
• sp³d Hybridization: Trigonal bipyramidal shape, e.g., SF₄, due to lone pair, becomes seesaw.
• sp³d² Hybridization: Octahedral shape or square planar shape, e.g., XeF₄.

The type of hybridization indicated for the central atom helps predict the arrangement of bonds and electron pairs around it.

Electron Pair Geometry

Electron pair geometry refers to the spatial arrangement of all pairs of electrons around a central atom, including both bonding pairs and lone pairs. This concept is determined by the number of electron groups around a central atom, where each group might be a single bond, multiple bond, or lone pair.
VSEPR theory (Valence Shell Electron Pair Repulsion) helps predict the electron pair geometries by assuming that electron pairs will arrange themselves to be as far apart as possible to minimize repulsion:

• Two electron groups: Linear geometry.
• Three electron groups: Trigonal planar geometry.
• Four electron groups: Tetrahedral geometry.
• Five electron groups: Trigonal bipyramidal geometry.
• Six electron groups: Octahedral geometry.

Differentiating between electron pair geometry and molecular shape is essential because lone pairs affect molecular shape but not the electron pair geometry.

Lone Pairs

Lone pairs refer to pairs of valence electrons that are not involved in chemical bonding within a molecule. These non-bonding pairs significantly influence the shape and properties of molecules.

• Presence and Impact: While lone pairs do not participate in forming bonds, they exert repulsive forces on adjacent electron pairs. This repulsion can lead to bond angles being smaller than expected, making the molecular shape different from the electron pair geometry.
• Influencing Geometry: For instance, a tetrahedral electron pair geometry with one lone pair results in a trigonal pyramidal molecular shape, like in NH₃.
• Effect on Polarity: Lone pairs can also contribute to molecular polarity, affecting how a molecule interacts with other substances and their solubility, boiling/melting points, and more.

Understanding the role of lone pairs aids in predicting molecular shape and physical properties such as reactivity, boiling/melting points, and intermolecular interactions.