Question

(a) Write one or more appropriate Lewis structures for the nitrite ion, \(\mathrm{NO}_{2}^{-}\). (b) With what allotrope of oxygen is it isoelectronic? (c) What would you predict for the lengths of the bonds in \(\mathrm{NO}_{2}^{-}\) relative to \(\mathrm{N}-\mathrm{O}\) single bonds and double bonds?

Short answer

The nitrite ion, \(\mathrm{NO}_{2}^{-}\), has a Lewis structure with one single bond and one double bond between the nitrogen and oxygen atoms, with delocalized electrons causing resonance. It is isoelectronic with the ozone (O\(_{3}\)) molecule. The bond lengths in \(\mathrm{NO}_{2}^{-}\) are expected to be shorter than a \(\mathrm{N}-\mathrm{O}\) single bond but longer than a \(\mathrm{N}-\mathrm{O}\) double bond due to electron delocalization.

Step-by-step solution

Determine the total number of valence electrons for \(\mathrm{NO}_{2}^{-}\)

First, we need to determine the total number of valence electrons available for \(\mathrm{NO}_{2}^{-}\). Nitrogen has 5 valence electrons, each oxygen atom has 6, and the extra negative charge contributes an additional electron. Total valence electrons = \(5 + 2 \times 6 + 1 = 18\)

Draw Lewis structures

With 18 valence electrons, one possible Lewis structure for \(\mathrm{NO}_{2}^{-}\) involves Nitrogen forming a single bond with one Oxygen atom and a double bond with the other Oxygen atom. The Lewis structure is: ``` O = N - O^(-) ``` Here, the central nitrogen atom has a total of 6 valence electrons (4 from bonding and 2 as a lone pair).

Identify the isoelectronic allotrope of oxygen

Isoelectronic species have the same number of electrons. The nitrite ion, as we determined earlier, has 18 electrons. Therefore, we need to find an allotrope of oxygen with 18 electrons as well. The O\(_{2}\) molecule has a total of 12 electrons (6 from each oxygen atom). For the allotrope of oxygen to have 18 electrons, we need a molecule with 3 oxygen atoms, which is the ozone (O\(_{3}\)) molecule. Thus, nitrite ion is isoelectronic with the ozone molecule.

Predict bond lengths in \(\mathrm{NO}_{2}^{-}\)

To predict the bond lengths in \(\mathrm{NO}_{2}^{-}\), we should consider the resonance between the single-bonded and double-bonded oxygen atoms. The electrons in the \(\mathrm{N}-\mathrm{O}\) bonds are delocalized, meaning that each bond is a hybrid of a single and double bond. This causes the bond lengths to fall somewhere between a typical single bond and double bond. Thus, we can predict that the bond lengths in \(\mathrm{NO}_{2}^{-}\) will be shorter than a \(\mathrm{N}-\mathrm{O}\) single bond but longer than a \(\mathrm{N}-\mathrm{O}\) double bond.

Key concepts

Valence Electrons

Understanding valence electrons is crucial when studying the behavior of atoms in chemical reactions. Valence electrons are the outermost electrons of an atom and are responsible for forming bonds with other atoms. For example, in the nitrite ion (NO_2^-), determining the total number of valence electrons is the first step in drawing the Lewis structure. Nitrogen (N) has 5 valence electrons in its outer shell, while oxygen (O) has 6. Since the nitrite ion carries an extra negative charge, it has an additional electron, adding up to 18 valence electrons in total.These electrons are then allocated around the nitrogen and oxygen atoms to form bonds and fulfill the octet rule, which states that atoms tend to form bonds until they have eight electrons in their valence shell, mirroring the electron configuration of noble gases.

Distributing the valence electrons is the foundation for creating the Lewis structure, a visual representation showcasing the arrangement of these electrons in the molecule.

Isoelectronic Species

Isoelectronic species are atoms, ions, or molecules that have the same number of electrons. Identifying isoelectronic species allows chemists to draw analogies between different structures and predict properties like bonding patterns and molecular geometry. In the exercise, we see that the nitrite ion (NO_2^-) has 18 electrons, making it isoelectronic with ozone (O_3), an allotrope of oxygen. Both species have the same number of electrons although they contain different numbers of protons.Isoelectronic relationships can guide predictions about how molecules may interact with each other. These species do not necessarily have the same chemical properties, as the arrangement of electrons and the types of atoms present can influence reactivity and bonding. Nonetheless, understanding isoelectronic relationships is a helpful tool in the field of chemistry.

Resonance

Resonance is a concept that describes the delocalization of electrons within certain molecular structures. A resonance structure occurs when more than one valid Lewis structure can be drawn for a molecule. The actual structure of the molecule is a hybrid or an average of these resonance structures. This phenomenon is particularly important in understanding the bonding and stability of molecules.In the case of the nitrite ion (NO_2^-), resonance allows the nitrogen atom to form bonds with both oxygen atoms—each bond having characteristics of both single and double bonds. This results in a delocalization of the N-O bonding electrons across the molecule. Resonance structures help explain why the actual bond lengths in the nitrite ion are equivalent and are in between the typical lengths of single and double N-O bonds.

Bond Lengths

Bond length refers to the distance between the nuclei of two bonded atoms. It is determined by the type of bond (single, double, triple) and the sizes of the atoms involved. In general, the more electrons shared between atoms, the stronger and shorter the bond becomes. A single bond is longer and weaker than a double bond, which is, in turn, longer and weaker than a triple bond.For the nitrite ion (NO_2^-), the bond lengths of the N-O bonds are not identical to typical single or double bonds due to the resonance effect. The actual bond length in the nitrite ion is expected to be intermediate between the two, shorter than a typical N-O single bond but longer than a typical double bond. This is because the molecule’s electron density is distributed over the two N-O bonds, causing a partial double-bond character in both. By understanding bond lengths, scientists can make educated guesses about the stability and reactivity of a molecule.