Test on Molecular shapes and VSEPR theory

Question 1: The molecular geometry of any molecule with a tetrahedral electron domain geometry will always be tetrahedral.

A. Yes

B. No

Explanation: When a central atom has four electron domains, they arrange themselves in a tetrahedral electron domain geometry. However, the molecular geometry can vary depending on the number of lone pairs. For example, if there are four bonding pairs (like CH₄), the molecular geometry is tetrahedral. But if there are three bonding pairs and one lone pair (like NH₃), the molecular geometry is trigonal pyramidal. If there are two bonding pairs and two lone pairs (like H₂O), the molecular geometry is bent.

Question 2: Which of the following statements identifies a common mistake or misconception when applying VSEPR theory?

A. Assuming that electron domain geometry and molecular geometry are always the same.

B. Failing to draw an accurate Lewis structure before predicting geometry.

C. Ignoring the fact that lone pairs exert greater repulsive forces than bonding pairs.

D. Believing that molecular geometry is irrelevant to a molecule's physical properties.

Explanation: According to the study materials, common mistakes include confusing electron domain geometry with molecular geometry, making errors in Lewis structures, and ignoring the greater repulsion of lone pairs. The statement that molecular geometry is irrelevant to a molecule's physical properties is incorrect, as the material states it dictates a molecule's reactivity, polarity, and boiling point.

Question 3: A molecule with polar bonds can be nonpolar if its symmetrical geometry leads to the cancellation of bond dipoles.

A. Yes

B. No

Explanation: If the bond dipoles cancel each other out due to the molecule's symmetrical geometry, the molecule will be nonpolar, even if it contains polar bonds. This occurs in molecules with symmetrical geometries like linear or tetrahedral where all terminal atoms are identical.

Question 4: Carbon dioxide is a polar molecule due to its linear geometry.

A. Yes

B. No

Explanation: Carbon dioxide has a linear geometry where its two opposing C=O bond dipoles cancel out, making the molecule nonpolar, not polar.

Question 5: Based on the description of its Lewis structure, which indicates no lone pairs on the central carbon atom, what is the polarity of methane (CH₄)?

A. Methane is polar because the C-H bonds are slightly polar.

B. Methane is nonpolar because its symmetrical tetrahedral geometry causes any bond dipoles to cancel out.

C. Methane is polar due to the presence of 4 hydrogen atoms.

D. Methane's polarity cannot be determined solely from its Lewis structure.

Explanation: The study materials state that methane's Lewis structure shows a central carbon atom with no lone pairs, leading to a symmetrical tetrahedral molecular geometry. Although the C-H bonds are slightly polar, this symmetrical geometry ensures that the bond dipoles cancel out, making methane a nonpolar molecule. Therefore, option B is correct.