Question

Arrange the following \(\mathrm{ACl}_{n}\) species in order of decreasing \(\mathrm{Cl}-\mathrm{A}-\mathrm{Cl}\) bond angles: \(\mathrm{SCl}_{2}, \mathrm{OCl}_{2}, \mathrm{PCl}_{3}, \mathrm{SiCl}_{4}, \mathrm{SiCl}_{6}^{2-}\)

Short answer

SiCl₄ > OCl₂ > PCl₃ > SCl₂ > SiCl₆²⁻.

Step-by-step solution

Understand the Problem

Identify the species involved and recognize that they are different molecules or ions containing chlorine atoms bonded to a central atom (A).

Determine the Electron Geometry

Identify the electron geometry for each species based on the Valence Shell Electron Pair Repulsion (VSEPR) theory. This involves counting the regions of electron density around the central atom (including bonds and lone pairs).

SCl₂ (Sulfur Dichloride)

Sulfur has 6 valence electrons + 2 from each chlorine atom. There are 4 regions of electron density (2 bonds, 2 lone pairs). The electron geometry is tetrahedral, but the molecular shape is bent. Approximate bond angle: 104.5°.

OCl₂ (Oxygen Dichloride)

Oxygen has 6 valence electrons + 2 from each chlorine atom. There are 4 regions of electron density (2 bonds, 2 lone pairs). The electron geometry is tetrahedral, but the molecular shape is bent. Approximate bond angle: 111° (greater than SCl₂ due to smaller size of O).

PCl₃ (Phosphorus Trichloride)

Phosphorus has 5 valence electrons + 3 from each chlorine atom. There are 4 regions of electron density (3 bonds, 1 lone pair). The electron geometry is tetrahedral, but the molecular shape is trigonal pyramidal. Approximate bond angle: 107°.

SiCl₄ (Silicon Tetrachloride)

Silicon has 4 valence electrons + 4 from each chlorine atom. There are 4 regions of electron density (4 bonds, 0 lone pairs). The electron geometry and molecular shape are both tetrahedral. Bond angle: 109.5°.

SiCl₆²⁻ (Hexachlorosilicate Ion)

Silicon still has 4 valence electrons but it is now bonded to 6 chlorine atoms and has gained 2 extra electrons. There are 6 regions of electron density (6 bonds, 0 lone pairs). The electron geometry is octahedral. Bond angle: 90°.

Arrange the Species

Arrange the species in order of decreasing bond angles: SiCl₄ (109.5°) > OCl₂ (~111°) > PCl₃ (~107°) > SCl₂ (~104.5°) > SiCl₆²⁻ (90°).

Key concepts

electron geometry

Electron geometry refers to the spatial arrangement of all electron regions (bonds and lone pairs) around a central atom, based on the Valence Shell Electron Pair Repulsion (VSEPR) theory. The VSEPR theory states that electron pairs repel each other and will therefore arrange themselves to be as far apart as possible.

Each type of electron geometry has characteristic angles between the bonds. Let's consider some examples from the exercise:

• Tetrahedral: When there are 4 regions of electron density, as seen in SiCl\(_4\), which has bond angles of 109.5°.
• Octahedral: Occurs when there are 6 regions of electron density, like in SiCl\(_6^{2-}\), featuring bond angles of 90°.

Understanding electron geometry is crucial because it serves as a foundation for predicting the shape of the molecule.

molecular shape

Molecular shape, or molecular geometry, arises from the electron geometry but only considers the positions of the atoms (ignoring lone pairs). The VSEPR theory helps predict the shape by minimizing repulsion between electron pairs.

Let's explore some molecular shapes using examples from the exercise:

• Bent: Seen in SCl\(_2\) and OCl\(_2\), both have a tetrahedral electron geometry but a bent molecular shape due to two lone pairs on the central atom.
• Trigonal Pyramidal: PCl\(_3\) has a tetrahedral electron geometry. With one lone pair, its shape is trigonal pyramidal.
• Tetrahedral: SiCl\(_4\) maintains both tetrahedral electron geometry and shape, having no lone pairs.
• Octahedral: SiCl\(_6^{2-}\) exhibits both an octahedral electron geometry and shape.

The molecular shape is essential for predicting chemical reactivity, physical properties, and bond angles.

bond angle comparison

Bond angle comparison is a crucial aspect of understanding molecular geometry. Bond angles are influenced by the electron geometry, the number of lone pairs, and the size of the central atom and surrounding atoms.

Comparing bond angles for the species from the exercise:

• SiCl\(_4\): With 109.5° as a standard tetrahedral angle.
• OCl\(_2\): Approximately 111°, slightly larger than SCl\(_2\) due to the smaller size of the oxygen atom creating larger repulsions between bonding pairs.
• PCl\(_3\): Around 107°, decreased because of one lone pair reducing the bond angle compared to an ideal tetrahedral.
• SCl\(_2\): About 104.5°, less than that for PCl\(_3\) due to an additional lone pair.
• SiCl\(_6^{2-}\): Having bond angles of 90°, characteristic of an octahedral geometry.

These angles show how the presence of lone pairs and the specific arrangement of atoms dictate the bond angles, essential for predicting the actual molecule's behavior and properties.