Problem 20 - Chemistry The Central Science In Si Units

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. (a) \(\mathrm{H}_{2} \mathrm{~S}\) (b) \(\mathrm{BCl}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{I}\) (d) \(\mathrm{CBr}_{4}\) (e) \(\mathrm{TeBr}_{4}\)

Step-by-Step Solution

Answer

Predict bond angles confidently for (a) H2S, (b) BCl3, (c) CH3I, (d) CBr4; uncertain for (e) TeBr4 due to lone pair.

Step 1: Determine molecular geometry of H2S

Hydrogen sulfide ( H_{2}S) has a bent molecular geometry, similar to H_{2}O, because sulfur is in the same group as oxygen. VSEPR theory predicts an angle of approximately 104.5 degrees. Due to lone pairs repulsion, the angle is slightly less than the tetrahedral angle of 109.5 degrees, so the bond angles can be predicted confidently.

Step 2: Determine molecular geometry of BCl3

Boron trichloride ( BCl_{3}) exists in a plane as a trigonal planar molecule because boron, the central atom, forms three bonded pairs and no lone pairs. This results in bond angles of 120 degrees. The prediction is confident since no lone pairs affect the bond angles.

Step 3: Determine molecular geometry of CH3I

Methyl iodide ( CH_{3}I) has a tetrahedral geometry. The central carbon atom forms four single bonds with no lone pairs. This results in bond angles of approximately 109.5 degrees. Since there are no lone pairs to cause additional repulsion, the angles can be confidently predicted.

Step 4: Determine molecular geometry of CBr4

Carbon tetrabromide ( CBr_{4}) also has a tetrahedral geometry, as carbon again forms four bonds without lone pairs. The bond angles are around 109.5 degrees. Similar to CH_{3}I, this prediction about bond angles is confident given the geometry.

Step 5: Determine molecular geometry of TeBr4

Tellurium tetrabromide ( TeBr_{4}) has a see-saw shape due to a lone pair next to four bonded pairs around the central tellurium atom (VSEPR AX4E1). The bond angles can be more challenging to predict accurately due to the presence of the lone pair, hence predictions are a bit uncertain.

Key Concepts

VSEPR Theory

The VSEPR Theory, standing for Valence Shell Electron Pair Repulsion, is pivotal in predicting the shapes of molecules. By considering the repulsion between electron pairs surrounding a central atom, this theory aids in determining molecular geometry. It works on the principle that electron pairs, both bonding and non-bonding, will arrange themselves in such a way as to minimize repulsion. This arrangement gives molecules their shape. Thus, understanding VSEPR is crucial for predicting both molecular shape and bond angles.
For example, in hydrogen sulfide ( {H}_{2}S), the bent geometry is predicted as a result of electron pair repulsion. The sulfur atom has two bonded pairs and two lone pairs. To minimize repulsion, these pairs position themselves to achieve the angle seen in water, leading to the confident prediction of bond angles close to 104.5 degrees.

Bond Angles

Bond angles refer to the angles formed between two covalent bonds that emanate from the same atom. These angles are determined by the specific molecular geometry and can vary based on several factors, including lone pair repulsion and the number of bonded atoms.

• In molecules like boron trichloride ( {BCl}_{3}), the planar trigonal shape results in predictable bond angles of 120 degrees without any lone pair influence.
• Tetrahedral molecules such as methyl iodide ( {CH}_{3}I) and carbon tetrabromide ( {CBr}_{4}) also have predictable bond angles of 109.5 degrees due to four evenly spaced bonded pairs.
• The case of tellurium tetrabromide ( {TeBr}_{4}) illustrates how lone pairs can create ambiguity, resulting in bond angles that are less predictable and require more detailed analysis.

Reliable predictions of bond angles provide insights into molecular shape and reactivity.

Tetrahedral Geometry

Tetrahedral geometry is a common molecular shape where a central atom is surrounded by four atoms positioned at the corners of a tetrahedron. This geometry occurs when there are four bonding pairs of electrons evenly spaced around the central atom, creating bond angles close to 109.5 degrees. This shape is highly symmetric and arises in several key molecules.

• In methyl iodide ( {CH}_{3}I) and carbon tetrabromide ( {CBr}_{4}), the central carbon atom forms four single bonds, leading to the formation of tetrahedral geometry.
• Tetrahedral molecules are well suited for predicting bond angles because their geometry is not influenced by lone pairs, thus making them stable and easily understood.

The symmetrical distribution of electron pairs in tetrahedral molecules results in equal repulsion forces, maintaining consistent bond angles.

Lone Pair Repulsion

Lone pair repulsion refers to the influence that lone pairs of electrons have on the geometry of a molecule. Unlike bonded pairs, lone pairs are concentrated in one location around the central atom and occupy more space. Consequently, they exert greater repulsive forces on other electron pairs. This leads to deviations in bond angles from those predicted by simple geometric shapes.

The presence of lone pair repulsion is evident in molecules like hydrogen sulfide ( {H}_{2}S) and tellurium tetrabromide ( {TeBr}_{4}). Here, the lone pairs push bonding pairs closer together, altering the predicted angles:

• In {H}_{2}S, lone pair repulsion reduces the bond angle from the tetrahedral 109.5 degrees to approximately 104.5 degrees.
• In {TeBr}_{4}, the lone pair contributes to a see-saw shape, creating more complex interactions and making bond angle predictions difficult.

Lone pair repulsion highlights the dynamic nature of molecular shapes, as non-bonding electrons can significantly influence molecular geometry.