Question

In which of the following \(\mathrm{AF}_{n}\) molecules or ions is there more than one \(\mathrm{F}-\mathrm{A}-\mathrm{Fbond}\) angle: \(\mathrm{SiF}_{4}, \mathrm{PF}_{5}, \mathrm{SF}_{4}, \mathrm{AsF}_{3} ?\)

Short answer

The molecules with more than one F-A-F bond angle are \(PF_5\) and \(SF_4\).

Step-by-step solution

Determine the Electron Domain Geometry (EDG) and Molecular Geometry (MG) for each molecule/ion

To determine EDG and MG, we can use the VSEPR (Valence Shell Electron Pair Repulsion) theory. The steps for this are: 1. Determine the central atom 2. Calculate the number of surrounding atoms and electron pairs around the central atom 3. Determine the shape/geometry based on VSEPR

SiF4

In SiF4, Si is the central atom. It has 4 surrounding F atoms and no lone pairs. This gives us a total of 4 electron domains (4 bonding pairs). Thus, the EDG is tetrahedral, and given that all surrounding atoms are the same, the MG is the same. It is a tetrahedral molecule with a single F-Si-F bond angle of 109.5°.

PF5

In PF5, P is the central atom, and it's surrounded by 5 F atoms with no lone pairs. This results in a total of 5 electron domains (5 bonding pairs). The EDG is trigonal bipyramidal, and the MG is the same. There are two types of bond angles for a trigonal bipyramidal molecule: axial F-P-F bond angles (two of them, 180° each) and equatorial F-P-F bond angles (three of them, 120° each).

SF4

In SF4, S is the central atom, with 4 F atoms bonded to it and one lone pair. This gives a total of 5 electron domains (4 bonding pairs and 1 lone pair). The EDG is trigonal bipyramidal, and the MG is "see-saw". Here, we have two S-F bond angles of approximately 120° and two F-S-F bond angles of approximately 180°.

AsF3

In AsF3, As is the central atom, and it's surrounded by 3 F atoms and one lone pair. This results in a total of 4 electron domains (3 bonding pairs and 1 lone pair). The EDG is tetrahedral, and the MG is trigonal pyramidal. The molecule has only one F-As-F bond angle of 107.7°.

Identify the molecules/ions with more than one F-A-F bond angle

Now that we've determined the molecular geometries, we can identify which molecules/ions have more than one F-A-F bond angle: - SiF4: Only one bond angle, not suitable. - PF5: More than one bond angle (axial and equatorial), suitable. - SF4: More than one bond angle (approximately 120° and 180° angles), suitable. - AsF3: Only one bond angle, not suitable. Therefore, the molecules with more than one F-A-F bond angle are \(PF_5\) and \(SF_4\).

Key concepts

Molecular Geometry

Understanding molecular geometry is crucial for determining how molecules will interact with one another, their reactivity, boiling and melting points, and even how they might smell or taste. In essence, molecular geometry is the three-dimensional arrangement of atoms within a molecule. It is determined by the number of electron pairs, both bonding and non-bonding (lone pairs), surrounding the central atom.

In our exercise, for example, the molecular geometry of SiF₄ is tetrahedral because the silicon atom is surrounded by four fluorine atoms with no lone pairs. This symmetrical shape means all F-Si-F bond angles are equal to 109.5°. On the other hand, SF₄ has a 'see-saw' shape due to the presence of a lone pair which distorts the ideal tetrahedral angle, leading to F-S-F bond angles that are not equivalent. This is why understanding molecular geometry is essential in predicting the behavior of molecules.

Electron Domain Geometry

The concept of electron domain geometry (EDG) is intimately linked to the VSEPR theory. It refers to the spatial arrangement of both the bonding and the lone pair electron domains around the central atom. By 'electron domain,' we mean a region where electrons are most likely to be found. Each bond, no matter if it's single, double, or triple, counts as one electron domain, and each lone pair is also one domain.

Electron domains repel each other and try to get as far apart as possible. In PF₅, the phosphorus is surrounded by five bonding pairs of electrons, which arrange themselves in a trigonal bipyramidal fashion as per EDG. This means there are two different types of bond angles, axial and equatorial, due to the three-dimensional spacing of the electron domains around the central atom.

Bond Angles

The measurement of the angle between two adjacent bonds in a molecule is known as a bond angle. These angles give us specific insights into the molecular geometry of a substance. The VSEPR theory helps predict the bond angles based on the repulsion between electron domains—representing both lone pairs and bonded atoms.

For instance, the ideal tetrahedral bond angle is 109.5°, as seen in SiF₄. However, the presence of lone pairs like in AsF₃ reduces the bond angle to 107.7° because lone pairs exert more repulsion than bonding pairs. In a molecule like PF₅, the axial and equatorial F-P-F bond angles are 180° and 120°, respectively, because of the trigonal bipyramidal EDG. Hence, comprehending bond angles provides a foundation for understanding broader molecular behavior and structure.