Video Transcript
The Lewis structure of a nitrite ion, NO2 1-, is shown in the diagram. What is the shape of this molecule?
The question is asking us to determine the three-dimensional shape of the nitrite ion. We can use the V-S-E-P-R model or VSEPR model to do so. VSEPR stands for valence shell electron pair repulsion. It is a model used to predict three-dimensional shapes of polyatomic molecules or ions by assuming the electrostatic repulsion between pairs of valence electrons will be minimized. Let’s apply this to a molecule like methane, or CH4.
While we draw the Lewis structure for methane in two dimensions, like this, with the carbon and hydrogen bonds each 90 degrees away from one another, the VSEPR model tells us that this is not with the three-dimensional shape would be like. Instead, the negatively charged domains around the central atom will situate in a way around the central carbon to minimize the repulsion, which is about 109.5 degrees apart. The methane molecule takes on a shape that appears like a tetrahedron, and thus the shape is called tetrahedral.
Let’s now use the VSEPR model to identify the three-dimensional shape of the nitrite ion. To do this, we must first identify the central atom in the structure. Next, we will count the number of bonding domains on the central atom. And finally, we will count the number of nonbonding domains on the central atom. To identify the central atom of the nitrite ion, we will use the Lewis structure shown. We can see that nitrogen has been drawn in the center with the other atoms present bonded to it. So, nitrogen is the central atom of this molecule. Now, let’s count the number of bonding domains on our nitrogen.
A bonding domain can be defined as an electron domain where bonding occurs. We can think of it as a site on the central atom where valence electrons are being shared with an outer atom. We can see that nitrogen has two sites where bonding is occurring with the oxygen atoms. This double bond is still considered one bonding domain since it is still only one site where bonding is happening between two atoms. There are two bonding domains present.
Finally, let’s count the number of nonbonding domains on our central atom. A nonbonding domain can also be called a lone pair, which is a pair of valence electrons not shared in a covalent bond. We can see in the Lewis structure that electrons represented as dots are present only in the valence shell of the atom on which they are drawn. These two electrons are present only in the valence shell of the nitrogen atom and are not shared in a covalent bond. These two electrons are a lone pair belonging only to nitrogen. Thus our central atom has one lone pair or, in other words, one nonbonding domain.
With this information in mind and using the VSEPR model, we can now work out the shape of the nitrite ion. We will focus on nitrogen as our central atom, which we found to have two bonding domains and one nonbonding domain. VSEPR model tells us that the repulsion between these domains will result in a three-dimensional shape that maximizes the distance between these negatively charged bonding and nonbonding domains, giving the nitrite ion a bent shape with bond angles of around 118 degrees. Thus, the shape of the nitrite ion is bent.
