to those you would Table 1.1: Bond Angles of Selected Molecules Bond-line Structure Approximate Bond Angle C. 180 :NEC-H H-Be-H :0: H 120 H. H. H. H. Critical Thinking Questions Use VSEPR to explain why the ZHBH bond angle of BH3 is 120°. (Hint: What is one-third cs 360°?) 9. None to those you would Table 1.1: Bond Angles of Selected Molecules Bond-line Structure 9. H H-N: H-Be-H H H a. : NECH b. H C. H :O: H H Critical Thinking Questions Use VSEPR to explain why the ZHBH bond angle of BH3 is 120°. (Hint: What is one-third of 360°?) H C 10. Both the ZHCH and ZHCO bond angles of Hâ‚‚CO (formaldehyde) are very close to 120°, but one is slightly smaller than the other. Predict which is smaller, and explain your reasoning. :C- Ð�’ 11. Use VSEPR to assign a value of “close to 180°” or “close to 120°” to each bond angle marked with a dotted line. (These angles are drawn as either 90° or 180°, but may be another value.) H H -H H 12. Consider the following flat drawing of methane (CH4). Approximate Bond Angle : N= 180 120 :01H IP HCH H- HHH H ✪ ning snel does itmebl What is ZHCH bond angle implied by this drawing if you assume it is flat? Are the electron domains of this flat CH4 spread out as much as possible? H HC-H H Use model materials to make a model of CH4 (methane). If you assembled it correctly, the four bonds (bonding electron domains) of your model will be 109.5° apart. d. In which representation, the drawing above or the model in your hand (circle one) are the H’s of CH4 more spread out around the central carbon?
The Correct Answer and Explanation is :
Answer to Critical Thinking Questions:
- VSEPR and the Bond Angle in BH₃: In BH₃ (borane), the central boron atom has three bonding pairs of electrons. According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, electron pairs in the valence shell of an atom will arrange themselves as far apart as possible to minimize repulsion. For BH₃, there are three electron pairs, and since they are arranged symmetrically around the boron atom, the bond angles between these pairs will be approximately 120°. The reasoning behind this is based on the geometry of a trigonal planar molecule, which has 120° bond angles between the bonding atoms. Since the total electron cloud around the boron atom forms a flat, equidistant triangular shape, the bond angles will be one-third of the total 360° around the central atom. This gives each bond angle in BH₃ as 120°.
- Bond Angles in H₂CO (Formaldehyde): In formaldehyde (H₂CO), both the C-H and C=O bond angles are very close to 120°. However, the C=O bond angle will be slightly smaller. This is because the oxygen atom in the C=O bond has a lone pair of electrons, which repels the bonding electron pairs more strongly, pushing the bonds slightly closer together and reducing the bond angle. In contrast, the C-H bonds are bonded with hydrogen atoms, which have no lone pairs, resulting in less repulsion. Therefore, the bond angle between the C-H bonds will be slightly larger than the C=O bond angle, but still very close to 120°.
- Bond Angles in CH₄ (Methane): The ideal bond angle in methane (CH₄) is 109.5°, which is due to the tetrahedral electron geometry around the central carbon atom. In the flat representation of methane (as shown in the question), the angles appear to be 90°, which is not correct because it does not represent the 3D spatial arrangement of the molecule. If you were to construct a model of CH₄ using model materials, the four hydrogen atoms would be arranged in a three-dimensional tetrahedral shape, with bond angles of 109.5° between them. This arrangement minimizes electron repulsion in accordance with VSEPR theory. Thus, the hydrogen atoms are more spread out in the three-dimensional model compared to the flat drawing, where the spatial separation is not accurate. In the flat drawing, the H atoms seem to be placed in a 2D plane, resulting in bond angles that are not representative of the true 3D structure.
In conclusion, the VSEPR theory helps explain the bond angles in these molecules by considering electron pair repulsion. The difference in bond angles between molecules like BH₃ and H₂CO can be attributed to factors such as lone pairs and the differences in atomic size and electron cloud distribution. For CH₄, the three-dimensional geometry is crucial for understanding the true bond angles, which are 109.5°, as shown by the model. The flat drawing fails to show this spread, highlighting the importance of considering the 3D structure when applying VSEPR theory.