Question

What is resonance? Give three examples of molecules or ions that exhibit resonance, and draw Lewis structures for each of the possible resonance forms.

Short answer

Resonance is a phenomenon where a molecule or ion exhibits multiple valid Lewis structures, representing different electron configurations that contribute to the overall electronic structure. Three examples of molecules or ions exhibiting resonance include Ozone (O3), Nitrate ion (NO3-), and Benzene (C6H6). Ozone resonance structures: \( \chemfig{O=[:-90]O-[:-180]O} \) and \( \chemfig{O-[:-90]O=[:-180]O} \) Nitrate ion resonance structures: \( \chemfig{[::+60]O-[:-60]N(=[::60]O)([::-60]O-)} \), \( \chemfig{[::+60]O-([::-60]N=[:-60]O)([::60]O-)} \), and \( \chemfig{[::+60]O([::-60]N-[:-60]O)(=[::60]O)} \) Benzene resonance structures: \( \chemfig{*6(-=-=-=)} \) and \( \chemfig{*6(=-=-=-)} \)

Step-by-step solution

Draw the first resonance structure for Ozone (O3)

For Ozone, the first resonance structure has a double bond between the central oxygen atom and one of the terminal oxygen atoms, and a single bond between the central oxygen atom and the other terminal oxygen atom. Also, add any lone pairs to complete the octet on each oxygen atom. \( \chemfig{O=[:-90]O-[:-180]O} \)

Draw the second resonance structure for Ozone (O3)

In the second resonance structure, the double bond switches to the other terminal oxygen atom. The remaining bond structure remains the same as in the first resonance form. \( \chemfig{O-[:-90]O=[:-180]O} \) Example 2: Nitrate ion (NO3-)

Draw the first resonance structure for Nitrate ion (NO3-)

In the first resonance structure of NO3-, the central nitrogen atom is single bonded to two of the oxygen atoms and double bonded to the third oxygen atom. Add any lone pairs to complete the octet on each atom and place a negative charge on the oxygen atom with the single bond. \( \chemfig{[::+60]O-[:-60]N(=[::60]O)([::-60]O-)} \)

Draw the second and third resonance structures for Nitrate ion (NO3-)

Rotate the double bond between nitrogen and oxygen atoms and place the negative charge accordingly to generate two more resonance forms. Second resonance form: \( \chemfig{[::+60]O-([::-60]N=[:-60]O)([::60]O-)} \) Third resonance form: \( \chemfig{[::+60]O([::-60]N-[:-60]O)(=[::60]O)} \) Example 3: Benzene (C6H6)

Draw the first resonance structure for Benzene (C6H6)

In the first resonance structure of benzene, draw a hexagonal carbon ring with alternate single and double carbon-carbon bonds. Attach a hydrogen atom to each carbon atom. \( \chemfig{*6(-=-=-=)} \)

Draw the second resonance structure for Benzene (C6H6)

For the second resonance structure, simply shift the position of the double bonds within the carbon ring. \( \chemfig{*6(=-=-=-)} \)

Key concepts

Lewis Structures

Understanding Lewis structures is a fundamental skill in chemistry—it's a way of representing the arrangement of electrons in a molecule. Lewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist.

A key point here is that electrons are arranged in a way to satisfy the octet rule, which states that atoms tend to bond in such a way that each atom has eight electrons in its valence shell. Lone pairs or non-bonding pairs are also indicated in Lewis structures. Understanding Lewis structures helps us predict the shape and reactivity of molecules, making them an essential tool in predicting chemical behavior.

Chemical Bonding

The concept of chemical bonding is crucial for explaining how atoms come together to form molecules. There are several types of chemical bonds, including ionic, covalent, and metallic bonds.

Covalent bonding, where atoms share electron pairs, is particularly important when discussing resonance in molecules like ozone or benzene. The overlapping of atomic orbitals allows for the distribution of electron density across the molecule that can't be represented by a single Lewis structure. The delocalization of these electrons can lead to increased stability and is a key factor in understanding resonance structures.

Molecular Geometry

The shape of a molecule, or molecular geometry, is another aspect deeply connected with chemical bonding and Lewis structures. The geometry is determined by the spatial arrangement of bonds and lone pairs around a central atom.

VSEPR (Valence Shell Electron Pair Repulsion) theory is used to predict molecular geometry by assuming that electron pairs around an atom repel each other and will arrange themselves as far apart as possible. This helps us understand the three-dimensional structure of a molecule, which directly influences its physical and chemical properties.

Ozone (O3) Structure

The ozone (O₃) molecule is a great example of resonance. It consists of three oxygen atoms where one atom forms a double bond with one oxygen and a single bond with the other.

But this doesn't account for the true distribution of electrons. The double bond can switch between the outer oxygen atoms, which is shown by drawing multiple resonance structures. Through these representations, we illustrate the phenomenon where the actual molecular structure is a hybrid of the individual resonance forms, leading to a delocalized electron cloud within the molecule.

Nitrate Ion (NO3-) Structure

Nitrate ion (⁻NO₃) is another classic case of resonance. The central nitrogen atom is bonded to three oxygen atoms, with one double bond and two single bonds.

Since all oxygen atoms are equivalent, the double bond can be with any of the oxygen atoms; thus, we have three resonance structures. The actual distribution of electrons in nitrate is a blend of these structures, leading to equal bond lengths among nitrogen-oxygen bonds. This delocalization contributes to the stability of the nitrate ion.

Benzene (C6H6) Structure

Benzene (C₆H₆) is a prime example of resonance in organic chemistry. It's composed of a ring of six carbon atoms, with alternating single and double bonds when depicted in Lewis structures.

However, in reality, these bonds are delocalized, creating a ring of uniform bond length and strength, different from any individual resonance structure. The concept here is that the delocalized electrons around the carbon ring create a more stable molecule than the alternating single and double bonds suggest.