Video Transcript
The following diagram shows a molecule of hydrazine. What is the N-N-H bond angle in hydrazine most likely to be? (A) 90 degrees, (B) 104.5 degrees, (C) 107 degrees, (D) 109.5 degrees, or (E) 120 degrees.
To answer this question, we can use the VSEPR model, which stands for valence shell electron pair repulsion. This model is used to predict three-dimensional shapes of polyatomic molecules or ions by assuming the electrostatic repulsion between pairs of electrons will be minimized. Using this model, we can also predict the bond angles in the molecule.
Let’s apply this model to a common example, such as methane. While we draw the two-dimensional Lewis structure like this with bond angles of 90 degrees, the VSEPR model tells us that this is not what the three-dimensional shape would be like. Instead, the negatively charged electron domains around the central carbon atom will situate in a way to minimize the electrostatic repulsion between them, and thus maximizing the space. This results in bond angles of approximately 109.5 degrees.
Let’s apply this information to the hydrazine molecule. Using the Lewis structure for hydrazine, we can apply the VSEPR model to determine the bond angle between the nitrogen-to-nitrogen bond and the nitrogen-to-hydrogen bond. We can focus on one of the nitrogen atoms as the central atom. As we saw with methane, there are four electron domains surrounding the central atom. Three of these electron domains are bonding domains, as they are a pair of valence electrons shared between two atoms. And one domain is a nonbonding domain, also called a lone pair.
A lone pair is a pair of valence electrons that are not shared in a covalent bond. This lone pair of electrons will affect the bond angles in the structure. We might expect bond angles of 109.5 as we saw with methane, since this central atom also has four electron domains surrounding it. However, the lone pair will generate more repulsion and will cause a smaller bond angle. Because of this additional repulsion, the bond angle will be approximately 107 degrees.
Therefore, the nitrogen-to-nitrogen-to-hydrogen bond angle in hydrazine is most likely to be answer choice (C), 107 degrees.
