Problem 112 - Chemistry An Atoms First Approach

Question

Predict the molecular structure for each of the following. (See Exercises 25 and \(26 .\) ) a. \(\mathrm{BrFI}_{2}\) b. \(\mathrm{XeO}_{2} \mathrm{F}_{2}\) c. \(\operatorname{TeF}_{2} \mathrm{Cl}_{3}^{-}\) For each formula there are at least two different structures that can be drawn using the same central atom. Draw all possible structures for each formula.

Step-by-Step Solution

Answer

The molecular structures for the given formulas are as follows: a. \(\mathrm{BrFI}_{2}\): T-shaped with two possible structures (Br as the central atom, with either 2 I atoms and 1 F atom, or 1 I atom and 2 F atoms bonded to it). b. \(\mathrm{XeO}_{2} \mathrm{F}_{2}\): Square planar with two possible structures (Xe as the central atom, with either O and F atoms in opposite corners or adjacent positions). c. \(\mathrm{TeF}_{2} \mathrm{Cl}_{3}^{-}\): See-saw shaped with three possible structures (Te as the central atom, with either 2 F and 3 Cl atoms, 1 F and 4 Cl atoms, or 2 F adjacent and 3 Cl atoms in an octahedral geometry).

Step 1: a. Predicting the molecular structure of \(\mathrm{BrFI}_{2}\)

Step 1: Identify the central atom In this molecule, the central atom is Br (bromine). Step 2: Determine the number of bonding and lone pairs around the central atom Br has 7 valence electrons. Each one of the Iodine (I) atoms is bonded to Br, and each of the Fluorine (F) atoms is bonded to Br. So there are a total of 3 bonding electron pairs, and 7 - 3 = 4 non-bonding (lone) electron pairs. Step 3: Predict the molecular structure based on VSEPR theory With 3 bonding electron pairs and 1 lone electron pair around the central atom (Br), the molecular geometry will be T-shaped. Step 4: Draw all possible structures for BrFI2 using the same central atom, Bromine There are two possible structures for BrFI2: 1. Br as the central atom with 2 I atoms and 1 F atom bonded to it. 2. Br as the central atom with 1 I atom and 2 F atoms bonded to it. In both cases, the structure is T-shaped with one lone pair on the central Br atom.

Step 2: b. Predicting the molecular structure of \(\mathrm{XeO}_{2} \mathrm{F}_{2}\)

Step 1: Identify the central atom In this molecule, the central atom is Xe (xenon). Step 2: Determine the number of bonding and lone pairs around the central atom Xe has 8 valence electrons. With 2 Oxygen (O) atoms and 2 Fluorine (F) atoms bonded to it, there are 4 bonding electron pairs, and 8 - 4 = 4 non-bonding (lone) electron pairs. Step 3: Predict the molecular structure based on VSEPR theory With 4 bonding electron pairs and 2 lone electron pairs around the central atom (Xe), the molecular geometry will be square planar. Step 4: Draw all possible structures for XeO2F2 using the same central atom, Xenon There are two possible structures for XeO2F2, with Xe (central atom) forming bonds to 2 O and 2 F atoms: 1. O and F atoms in opposite corners of the square planar geometry. 2. O and F atoms adjacent in the square planar geometry. In both cases, the structure is square planar with two lone pairs on the central Xe atom.

Step 3: c. Predicting the molecular structure of \(\operatorname{TeF}_{2} \mathrm{Cl}_{3}^{-}\)

Step 1: Identify the central atom In this molecule, the central atom is Te (tellurium). Step 2: Determine the number of bonding and lone pairs around the central atom Te has 6 valence electrons. With 2 Fluorine (F) atoms and 3 Chlorine (Cl) atoms bonded to it, there are 5 bonding electron pairs, and 6 + 1 (due to the negative charge) - 5 = 2 non-bonding (lone) electron pairs. Step 3: Predict the molecular structure based on VSEPR theory With 5 bonding electron pairs and 1 lone electron pair around the central atom (Te), the molecular geometry will be see-saw shaped. Step 4: Draw all possible structures for TeF2Cl3- using the same central atom, Tellurium There are three possible structures for TeF2Cl3-, with Te (central atom) forming bonds to 2 F atoms and 3 Cl atoms: 1. Te is the central atom with 2 F atoms and 3 Cl atoms in an octahedral geometry, resulting in a see-saw shaped molecular structure. 2. Te is the central atom with 1 F atom and 4 Cl atoms in an octahedral geometry, resulting in a see-saw shaped molecular structure. 3. Te is the central atom with 2 F atoms adjacent and 3 Cl atoms in an octahedral geometry, resulting in a see-saw shaped molecular structure. In all cases, the structure is see-saw shaped with one lone pair on the central Te atom.

Key Concepts

Molecular Geometry

Molecular geometry is a key concept in chemistry that describes the arrangement of atoms within a molecule. It helps us understand how molecules form specific structures based on their chemical bonds. This is important because the shape of a molecule can affect its physical and chemical properties, such as polarity, reactivity, and color. The VSEPR theory, which stands for Valence Shell Electron Pair Repulsion, is commonly used to predict molecular geometry.

According to VSEPR theory, electron pairs around a central atom arrange themselves in such a way as to minimize repulsion. This means that both bonding pairs (which form bonds between atoms) and lone pairs (non-bonded electron pairs) have a role in determining a molecule's shape.

Some common molecular geometries include:

• T-shaped, as seen in BrFI2, where the central atom has more electron pairs in one plane forming a 'T' shape.
• Square planar, such as in XeO2F2, where the central atom forms a flat square shape with specific atoms.
• See-saw shaped, shown in TeF2Cl3-, where the geometry resembles a see-saw formation.

Understanding molecular geometry helps in visualizing and predicting how a molecule will behave in interactions and reactions.

Central Atom

The central atom is the focal point in a molecule where most of the bonds occur. It is usually the atom with the lowest electronegativity in the molecule, with a few exceptions depending on specific chemistry rules. The central atom is crucial as it is the core around which molecular geometry is determined.

For example, in the molecule \( ext{BrFI}_2\), bromine (Br) acts as the central atom. It is surrounded by iodine and fluorine atoms, dictating the T-shaped molecular geometry.

Similarly, in \( ext{XeO}_2 ext{F}_2\), xenon (Xe) is the central atom. Xenon forms bonds with oxygen and fluorine atoms to result in a square planar shape.

Tellurium (Te) is the central atom in the molecule \( ext{TeF}_2 ext{Cl}_3^- ext{.}\), acting as the nucleus forming connections with chlorine and fluorine atoms. The central atom's electron configuration often determines how bonding and lone pairs arrange themselves, which directly influences the molecular geometry and stability of the molecule.

Bonding Pairs

Bonding pairs refer to pairs of electrons shared between two atoms, forming a covalent bond. These pairs are integral in creating connections between atoms within a molecule and are key contributors to defining a molecule's shape.

For instance, in \( ext{BrFI}_2\), there are three bonding pairs with Br (bromine) forming bonds with both iodine (I) and fluorine (F) atoms. These shared electron pairs lead to the overall T-shaped geometry by influencing how electron pairs are distributed geometrically.

In \( ext{XeO}_2 ext{F}_2\), xenon (Xe) utilizes four bonding pairs to bind to the surrounding oxygen (O) and fluorine (F) atoms, leading to a square planar structure.

In \( ext{TeF}_2 ext{Cl}_3^-\), tellurium (Te) uses five bonding pairs to form connections with fluorine and chlorine atoms. Bonding pairs play a significant part in molecular geometry because they dictate how many atoms can be directly connected to the central atom, thus shaping the molecule's form.

Lone Pairs

Lone pairs, also known as non-bonding pairs, are pairs of valence electrons that are not shared with another atom and do not participate directly in bonding. Despite not making bonds, they have a substantial effect on the shape and geometry of molecules because they take up space and repel binding electron pairs.

In the \( ext{BrFI}_2\) molecule, bromine has lone pairs that influence its T-shaped geometry by pushing against the bonding pairs.

The \( ext{XeO}_2 ext{F}_2\) molecule features xenon with lone pairs, which helps it achieve a square planar formation by balancing the spatial arrangement of bonding pairs.

For \( ext{TeF}_2 ext{Cl}_3^-\), tellurium's lone pairs affect the molecular shape, resulting in a see-saw configuration. Lone pairs can significantly alter molecular geometry by dictating angles and positions, even though they don't form bonds with other atoms.