Problem 20 - Chemistry The Central Science

Question

In which of the following molecules can you confidently predict the bond angles about the central atom, and for which would you be a bit uncertain? Explain in each case. \((\mathbf{a}) \mathrm{H}_{2} \mathrm{S},(\mathbf{b}) \mathrm{BCl}_{3},(\mathbf{c}) \mathrm{CH}_{3} \mathrm{I},(\mathbf{d}) \mathrm{CBr}_{4,}(\mathbf{e}) \mathrm{TeBr}_{4}\)

Step-by-Step Solution

Answer

We can confidently predict the bond angles for BCl3 (trigonal planar), CBr4 (tetrahedral), and TeBr4 (trigonal bipyramidal) using the VSEPR theory. However, we cannot confidently predict the bond angles for H2S (bent) and CH3I (tetrahedral) due to the presence of lone pairs and differing electronegativities of surrounding atoms, respectively.

Step 1: Identify central atom and electron pairs

First, we need to identify the central atom in each molecule and determine the number of bonding and non-bonding electron pairs around it: (a) H2S - Central atom: S; 2 bonding pairs (H-S); 2 lone pairs (b) BCl3 - Central atom: B; 3 bonding pairs (B-Cl); 0 lone pairs (c) CH3I - Central atom: C; 3 bonding pairs (C-H); 0 lone pairs; 1 bonding pair(C-I) (d) CBr4 - Central atom: C; 4 bonding pairs (C-Br); 0 lone pairs (e) TeBr4 - Central atom: Te; 4 bonding pairs (Te-Br); 1 lone pair

Step 2: Predict molecular geometry using VSEPR

Now, we'll determine the molecular geometry based on the VSEPR theory: (a) H2S - 2 bonding pairs and 2 lone pairs - Bent molecular geometry (b) BCl3 - 3 bonding pairs and 0 lone pairs - Trigonal planar molecular geometry (c) CH3I - 3 C-H bonding pairs and 1 C-I bonding pair - Tetrahedral molecular geometry (d) CBr4 - 4 bonding pairs and 0 lone pairs - Tetrahedral molecular geometry (e) TeBr4 - 4 bonding pairs and 1 lone pair - Trigonal bipyramidal molecular geometry

Step 3: Identify molecules with uncertain bond angles

We can now identify the molecules with uncertain bond angles: (a) H2S - As there are two bonding pairs and two lone pairs, the electron pair repulsion is unequal, and the bond angle might be slightly less than the ideal angle for a bent geometry, which is \(104.5^{\circ}\). (c) CH3I - In the case of CH3I, there are unequal electron pair repulsion forces because of the difference in electronegativity of H and I. This makes the bond angle prediction uncertain.

Step 4: Conclusion

Based on the VSEPR theory and molecular geometry, we can confidently predict the bond angles for BCl3 (trigonal planar), CBr4 (tetrahedral), and TeBr4 (trigonal bipyramidal). However, we cannot confidently predict the bond angles for H2S (bent) and CH3I (tetrahedral) due to the presence of lone pairs and differing electronegativities of surrounding atoms, respectively.

Key Concepts

Molecular Geometry

Understanding molecular geometry is crucial for predicting properties like polarity, reactivity, and color of different compounds. Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The Valence Shell Electron Pair Repulsion (VSEPR) theory guides us in determining these shapes by assuming that electron pairs around a central atom will arrange themselves to minimize repulsion, giving us a predictable molecular structure.

For example, a molecule like BCl3, which has three bonding pairs and no lone pairs, adopts a trigonal planar geometry to minimize repulsion. On the other hand, H2S with two bonding pairs and two lone pairs forms a bent shape due to the additional space lone pairs require compared to bonding pairs. Predicting the geometry not only helps in visualizing the molecule but is also foundational in understanding its interaction with other molecules and its overall chemistry.

Bond Angles Prediction

Predicting bond angles is a bit like solving a spatial puzzle. Using the VSEPR theory, we can estimate the angles between bonded electron pairs. In a molecule with a tetrahedral layout such as CBr4, with equal repulsion among the four bonding electron pairs, the bond angles are roughly 109.5 degrees each. However, the presence of lone pairs or differences in the size and electronegativity of surrounding atoms can alter these angles.

Lone pair electrons occupy more space and can push bonding pairs closer together, resulting in smaller bond angles than in an ideal tetrahedral. This can be seen in a molecule like TeBr4, where the lone pair forces the Br-Te-Br bond angles in the equatorial plane to be less than the ideal 120 degrees. Predictions can become uncertain when these and other factors, like hybridization or the presence of multiple bonds, come into play.

Electron Pair Repulsion

Electron pair repulsion is the fundamental idea behind the VSEPR theory. It's the proverbial 'social distancing' at the atomic level. Electrons, being negatively charged, repel each other. This repulsion between valence electron pairs—whether bonding or non-bonding (lone pairs)—affects molecular geometry.

Bonding pairs, consisting of two electrons each, are shared between two atoms and are generally closer together than lone pairs, which are not shared and are only influenced by the nucleus of one atom. In our case of H2S, the lone pairs are a bit like extra guests at a dinner table—they need more room, leading to a bending of the structure away from the ideal linear geometry. The theory helps explain not only the shapes of molecules but also the angles between bonds, providing a framework for predicting many molecular properties and behaviors.