Question

Which of the following do not have resonance structures? (1) ${\mathrm{CO}}_3^2$ (2) ${\mathrm{NO}}_3$ (3) ${\mathrm{NH}}_3$ (4) ${\mathrm{BF}}_3$

Short answer

${\text{NH}}_3$ and ${\text{BF}}_3$ do not have resonance structures.

Step-by-step solution

Identify possible resonance structures

Resonance structures occur when there are multiple ways to arrange double and single bonds in a molecule. Examine each given molecule to determine if it has resonance structures.

Analyze ${\text{CO}}_3^{2-}$

${\text{CO}}_3^{2-}$ (carbonate ion) has three oxygen atoms bonded to a central carbon atom. It has multiple valid Lewis structures where the double bond can be placed between carbon and any one of the three oxygen atoms, indicating it has resonance structures.

Analyze ${\text{NO}}_3^{-}$

${\text{NO}}_3^{-}$ (nitrate ion) has three oxygen atoms bonded to a central nitrogen atom. Like carbonate, it has multiple valid Lewis structures where the double bond can be placed between nitrogen and any one of the three oxygen atoms, indicating it has resonance structures.

Analyze ${\text{NH}}_3$

${\text{NH}}_3$ (ammonia) has three hydrogen atoms bonded to a central nitrogen atom. There are no multiple bonding possibilities or electron pairs to move around leading to different structures. Thus, ${\text{NH}}_3$ does not have resonance structures.

Analyze ${\text{BF}}_3$

${\text{BF}}_3$ (boron trifluoride) has three fluorine atoms bonded to a central boron atom. Each fluorine is singly bonded to the boron, and there are no possibilities for different arrangement of bonds to create resonance structures. Thus, ${\text{BF}}_3$ does not have resonance structures.

Conclusion

The molecules that do not have resonance structures are ${\text{NH}}_3$ and ${\text{BF}}_3$.

Key concepts

molecular structure

Understanding the molecular structure of a compound is key to grasping many concepts in chemistry, including resonance structures. The term 'molecular structure' refers to the three-dimensional arrangement of atoms in a molecule. This arrangement affects many properties of the substance, such as its reactivity, polarity, phase of matter, color, magnetism, biological activity, and more.
In the context of our exercise, knowing the molecular structure of compounds like ${\text{CO}}_3^{2-}$ (carbonate ion) and ${\text{NO}}_3^{-}$ (nitrate ion) helps us understand why these molecules can have resonance forms. Conversely, the simple geometry of ${\text{NH}}_3$ (ammonia) and ${\text{BF}}_3$ (boron trifluoride) makes it clear why they cannot have resonance structures. Let's dive deeper into what makes a structure conducive to resonance.

Lewis structures

Lewis structures are a way to represent molecules using symbols for atoms and dots for electrons. These structures help us visualize the bonding between atoms as well as the lone pairs of electrons that may exist.
When writing Lewis structures, we follow a set of rules, such as ensuring that the total number of valence electrons is accounted for and that the octet rule, where atoms tend to have eight electrons in their valence shell, is satisfied when possible. For example:

• In ${\text{CO}}_3^{2-}$, we can draw several Lewis structures by moving the double bond between the carbon and one of the oxygen atoms.
• In ${\text{NO}}_3^{-}$, we can also shift the location of the double bond between the nitrogen and one of the three oxygen atoms, resulting in different valid resonance forms.
• For ${\text{NH}}_3$, since there are only single bonds between nitrogen and hydrogen, there aren't multiple valid ways to arrange these bonds.
• Similarly, ${\text{BF}}_3$ has only single bonds between boron and fluorine, thus no resonance is possible.

Understanding Lewis structures is important for determining if a molecule has resonance structures.

chemical bonding

Chemical bonding is the force that holds atoms together in molecules or compounds. There are different types of bonds, including covalent bonds, ionic bonds, and metallic bonds. Covalent bonds, where atoms share pairs of electrons, are especially relevant when discussing resonance structures.
Resonance occurs in such molecules where electrons can be shared between atoms in different configurations while still keeping the overall connectivity of the molecule the same. This is typically seen in molecules with conjugated systems, like ${\text{CO}}_3^{2-}$ and ${\text{NO}}_3^{-}$.
Ammonia ${\text{NH}}_3$, on the other hand, has single covalent bonds between nitrogen and hydrogen, and no delocalized electrons that can be shared among different locations, thus no resonance.
Boron trifluoride ${\text{BF}}_3$ also has single covalent bonds between boron and fluorine with no possibility of resonance, as there are no other overlapping p-orbitals that could support the sharing of electrons in different configurations.
Resonance structures do not alter chemical bonding but show alternative ways of visualizing electron distribution, ultimately highlighting how certain molecules can stabilize through electron delocalization.