Question

Predict the molecular structure (including bond angles) for each of the following. (See Exercises 115 and 116.) a. \(\mathrm{XeCl}_{2}\) b. \(\mathrm{ICl}_{3}\) c. \(\mathrm{TeF}_{4}\) d. \(\mathrm{PCl}_{5}\)

Short answer

The molecular structures and bond angles for the given molecules are as follows: a. XeCl2: The molecular geometry is bent, with bond angles of approximately 120 degrees. b. ICl3: The molecular geometry is T-shaped, with bond angles of approximately 90 degrees. c. TeF4: The molecular geometry is see-saw, with bond angles of approximately 120 degrees and 90 degrees. d. PCl5: The molecular geometry is trigonal bipyramidal, with bond angles of approximately 120 degrees in the equatorial plane and 90 degrees between the axial and equatorial positions.

Step-by-step solution

Molecule XeCl2

Step 1: Identify the central atom and count the number of electron domains around it Xe is the central atom, and it has two Cl atoms bonded to it. Xe has 8 valence electrons, and 2 of these electrons form covalent bonds with the 2 Cl atoms. So, the central atom has 3 electron domains: 2 bonding domains (one for each Cl atom) and 1 nonbonding domain (3 lone pairs). Step 2: Determine the molecular geometry using the VSEPR table With 3 electron domains (2 bonding and 1 nonbonding), the molecular geometry of XeCl2 is bent, and the bond angles are approximately 120 degrees.

Molecule ICl3

Step 1: Identify the central atom and count the number of electron domains around it I is the central atom, and it has three Cl atoms bonded to it. I has 7 valence electrons, and 3 of these electrons form covalent bonds with the 3 Cl atoms, leaving 2 lone pairs on the I atom. So, there are 5 electron domains: 3 bonding domains (one for each Cl atom) and 2 nonbonding domains (2 lone pairs). Step 2: Determine the molecular geometry using the VSEPR table With 5 electron domains (3 bonding and 2 nonbonding), the molecular geometry of ICl3 is T-shaped, and the bond angles are approximately 90 degrees.

Molecule TeF4

Step 1: Identify the central atom and count the number of electron domains around it Te is the central atom, and it has four F atoms bonded to it. Te has 6 valence electrons, and 4 of these electrons form covalent bonds with the 4 F atoms, leaving 1 lone pair on the Te atom. So, there are 5 electron domains: 4 bonding domains (one for each F atom) and 1 nonbonding domain (1 lone pair). Step 2: Determine the molecular geometry using the VSEPR table With 5 electron domains (4 bonding and 1 nonbonding), the molecular geometry of TeF4 is see-saw, and the bond angles are approximately 120 degrees and 90 degrees.

Molecule PCl5

Step 1: Identify the central atom and count the number of electron domains around it P is the central atom, and it has five Cl atoms bonded to it. P has 5 valence electrons, and all of these electrons form covalent bonds with the 5 Cl atoms, leaving no lone pairs on the P atom. So, there are 5 electron domains, which are all bonding domains (one for each Cl atom). Step 2: Determine the molecular geometry using the VSEPR table With 5 electron domains (all bonding), the molecular geometry of PCl5 is trigonal bipyramidal, and the bond angles are approximately 120 degrees in the equatorial plane and 90 degrees between the axial and equatorial positions.

Key concepts

Molecular Geometry

Molecular geometry is the three-dimensional shape that is formed when atoms bond together in a molecule. This shape helps decide many of the molecule's properties, like how it fits with other molecules.
Knowing the molecular geometry can be really helpful, whether you're studying chemistry or looking at real-life applications like drug design.

To predict this shape, chemists use the VSEPR (Valence Shell Electron Pair Repulsion) theory.

• The VSEPR theory says that electron pairs around a central atom will arrange themselves as far apart as possible.
• This is because electrons repel each other, much like the same poles of a magnet.
• Using this theory, you can find out if a molecule is a linear, bent, trigonal planar, tetrahedral shape, or more simply by analyzing the number of bonding and lone electron pairs in its configuration.

In the examples from the exercise:

• \(\mathrm{XeCl}_{2}\) has a bent shape because it has 2 bond pairs and one lone pair.
• \(\mathrm{ICl}_{3}\) forms a T-shaped configuration due to its 3 bond pairs and 2 lone pairs.
• TeF4 is described as having a see-saw structure with 4 bond pairs and one lone pair of electrons.
• Finally, \(\mathrm{PCl}_{5}\) is trigonal bipyramidal, with all five pairs as bond pairs.

Electron Domains

Electron domains include both bonding and nonbonding electron pairs around the central atom. Understanding electron domains helps in predicting the molecular structure using VSEPR theory.
Each set of shared or unshared electron pairs counts as a single domain, regardless of how many electrons are involved.

Here’s how to count them:

• Bonding domains are formed by electrons that are shared between atoms, while nonbonding domains contain lone pairs, or electrons that are not shared with another atom.
• In the molecule XeCl2, the central atom has 3 electron domains: 2 bonding (to Cl atoms) and 1 nonbonding lone pair.
• For ICl3, there's a total of 5 electron domains, with 3 bonding domains and 2 nonbonding domains (lone pairs).
• TeF4 also has 5 electron domains, but this time 4 are bonding and 1 is nonbonding.
• PCl5's configuration features 5 electron domains, all being bonding domains.

Bond Angles

Bond angles are the angles between adjacent lines representing bonds. These angles help define the overall shape of the molecule.
By knowing the bond angles, you can learn a lot about the molecule’s geometry and potential interactions with other molecules.

Let's see some examples:

• For the molecule XeCl2, the bond angles are bent and approximately 120 degrees, due to the lone pair electron-domain repulsions.
• In ICl3, the T-shaped geometry results in bond angles around 90 degrees.
• TeF4's see-saw shape means it has two different approximate angles: one set of angles at 120 degrees and another at 90 degrees.
• For PCl5, the trigonal bipyramidal shape gives it bond angles of 120 degrees in the equatorial plane and 90 degrees between the axial and equatorial positions.

Central Atom Identification

The central atom in a molecule is usually the one that is least electronegative and can form multiple bonds. Recognizing the central atom is vital for predicting a molecule's shape since all the other atoms connect to it.
This identification helps in counting electron domains and predicting molecular geometry.

Here's a quick guide:

• In \(\mathrm{XeCl}_{2}\), the central atom is Xe, due to its ability to expand its valence shell to accommodate more than 8 electrons.
• For \(\mathrm{ICl}_{3}\), iodine (I) is the central atom, since it can form three bonds with Cl atoms.
• In \(\mathrm{TeF}_{4}\), tellurium (Te) serves as the central atom, again capable of accommodating multiple bonds.
• Finally, phosphorus (P) is the central atom in \(\mathrm{PCl}_{5}\), with its five valence electrons allowing it to form five bonds with Cl atoms.

Understanding the central atom involves combining the knowledge of chemical properties and practical application of electronegativity and valence concepts.