Question

In which of these molecules or ions does the presence of nonbonding electron pairs produce an effect on molecular shape? (a) \(\mathrm{CO}_{2},(\mathbf{b}) \mathrm{CH}_{2} \mathrm{Br}_{2,}(\mathbf{c}) \mathrm{OF}_{2},(\mathbf{d}) \mathrm{BCl}_{3},(\mathbf{e}) \mathrm{SF}_{6}\)

Short answer

Only molecule (c) OF₂ has nonbonding electron pairs that produce an effect on the molecular shape, resulting in a bent geometry.

Step-by-step solution

Identify the central atom for each molecule

For each molecule, we will first identify the central atom. The central atom is typically the least electronegative element and has the highest valence. In our case, we have: a) CO₂: C is the central atom b) CH₂Br₂: C is the central atom c) OF₂: O is the central atom d) BCl₃: B is the central atom e) SF₆: S is the central atom

Calculate the number of valence electrons and electron groups

Next, we will calculate the number of valence electrons for the central atom in each molecule and count the number of electron groups (lone pairs and bonding pairs) surrounding each central atom: a) CO₂: C has 4 valence electrons + 2 * (2 valence electrons for each O) = 8 valence electrons and 2 electron groups (2 double bonds) b) CH₂Br₂: C has 4 valence electrons + 2 * (1 valence electron for each H) + 2 * (7 valence electrons for each Br) = 20 valence electrons and 4 electron groups (2 single bonds and 2 single bonds) c) OF₂: O has 6 valence electrons + 2 * (7 valence electrons for each F) = 20 valence electrons and 4 electron groups (2 single bonds and 2 lone pairs) d) BCl₃: B has 3 valence electrons + 3 * (7 valence electrons for each Cl) = 24 valence electrons and 3 electron groups (3 single bonds) e) SF₆: S has 6 valence electrons + 6 * (7 valence electrons for each F) = 48 valence electrons and 6 electron groups (6 single bonds)

Determine molecular geometry using VSEPR Theory

Now, for each molecule, we will apply the VSEPR Theory to determine the molecular geometry. Take into consideration the electron group geometry as well: a) CO₂: 2 electron groups with no lone pairs give a linear geometry. b) CH₂Br₂: 4 electron groups with no lone pairs give a tetrahedral geometry. c) OF₂: 4 electron groups with 2 lone pairs give a bent geometry. d) BCl₃: 3 electron groups with no lone pairs give a trigonal planar geometry. e) SF₆: 6 electron groups with no lone pairs give an octahedral geometry.

Identify if nonbonding electron pairs affect molecular shape

Finally, we will identify which molecules have nonbonding electron pairs that affect their shape: a) CO₂: No lone pairs, linear geometry - No effect b) CH₂Br₂: No lone pairs, tetrahedral geometry - No effect c) OF₂: 2 lone pairs, bent geometry - Effect on molecular shape d) BCl₃: No lone pairs, trigonal planar geometry - No effect e) SF₆: No lone pairs, octahedral geometry - No effect So, only molecule (c) OF₂ has nonbonding electron pairs that produce an effect on the molecular shape.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It is essential to understand this concept because it helps predict the shape and properties of molecules. The shape of a molecule is impacted by the number and arrangement of electron pairs surrounding the central atom. Molecular geometry can vary greatly, allowing for shapes such as linear, bent, tetrahedral, trigonal planar, and octahedral, among others. This geometry is determined using the Valence Shell Electron Pair Repulsion (VSEPR) Theory, which postulates that electron pairs around a central atom will position themselves to minimize repulsion. For example, in the molecule CO₂, which is part of the exercise, the geometry is linear. This is because the central carbon atom has two electron groups forming double bonds with oxygen, which pushes the two oxygen atoms 180 degrees apart. On the other hand, OF₂ exhibits a bent shape due to the presence of two lone pairs on the oxygen, which causes the bonded fluorine atoms to be pushed closer together.

Nonbonding Electron Pairs

Nonbonding electron pairs, also known as lone pairs, are valence electrons that are not involved in bonding but belong exclusively to one atom. These lone pairs can have a significant influence on molecular geometry because they occupy space around the central atom and cause repulsion. The presence of nonbonding electron pairs is crucial in determining the molecular shape. They tend to repel bonding pairs more strongly than bonding pairs repel each other, often leading to a change in the expected geometry. A clear example from the exercise is in OF₂. Here, two lone pairs are present on the oxygen atom, leading to a bent geometry. The lone pairs push the bonded atoms closer together, changing the idealized tetrahedral angle of 109.5 degrees to a smaller angle around 104.5 degrees. Nonbonding electron pairs do not only affect shape but can also impact the polarity and reactivity of the molecule.

Central Atom Identification

Identifying the central atom is a key step in determining the molecular geometry and understanding the molecule's behavior. The central atom is generally the least electronegative atom in the molecule and is often the atom that forms the most bonds. For instance, in CO₂, CH₂Br₂, and SF₆, Carbon, Carbon, and Sulfur are the central atoms, respectively. These atoms are less electronegative compared to the other atoms in the compound, enabling them to serve as the core around which other atoms are arranged. When selecting a central atom, consider its ability to hold multiple bonds; this often correlates with having a larger atomic size or a higher valence electron count. In the examples given, after identifying the central atom, it becomes easier to count electron groups and apply VSEPR theory to predict molecular shapes accurately. Thus, accurately identifying the central atom sets the foundation for further analysis of molecular structure and properties.