Resonance Structures and Resonance Hybrids in Polyatomic Ions

Resonance Structures in Polyatomic Ions

Definition and Importance

Resonance structures are a set of two or more valid Lewis Dot Structures for polyatomic species that possess at least one double bond. These structures illustrate the delocalization of electrons within molecules or ions, especially when electrons can move between different positions, such as between a double bond and a lone pair.

• Resonance Structure: A representation where electrons are shown moving from either a double bond or lone pair to another position in the molecule.

• Double Sided Arrows (↔): Used to indicate that resonance structures are equivalent to each other.

• Composite/Resonance Hybrid: The actual structure of the molecule is a hybrid of all possible resonance structures, called the resonance hybrid.

• Dotted Line: In the resonance hybrid, a dotted line is drawn anywhere a double bond has been to show delocalization.

Example: The nitrite ion (NO2-) has two resonance structures, each with a double bond and a lone pair on different oxygen atoms.

$\begin{array}{c} \mathrm{O=N-O^{-} \leftrightarrow O^{-}-N=O} \end{array}$

Resonance Hybrid

Concept and Representation

The resonance hybrid is a composite of all major resonance structures. It represents the true electronic structure of the molecule, where electrons are delocalized over multiple atoms. In diagrams, dotted lines are used to indicate regions where double bonds are shared among atoms.

• Resonance Hybrid: Shows partial bonds and delocalized electrons.

• Application: Resonance hybrids are important for understanding molecular stability, reactivity, and properties.

Example: The resonance hybrid of NO2- is shown with dotted lines between nitrogen and both oxygen atoms.

$\begin{array}{c} \mathrm{O\cdots N \cdots O} \end{array}$

Examples of Resonance Structures

Carbonate Ion (CO32-)

The carbonate ion has three equivalent resonance structures, each with a double bond between carbon and a different oxygen atom. The negative charges are distributed among the oxygen atoms.

• Key Points:

  • Each resonance structure has one C=O double bond and two C–O single bonds.

  • The negative charge is located on the oxygen atoms with single bonds.

  • All three structures are equivalent.

Resonance Structures:

$\begin{array}{c} \mathrm{O^{2-}-C=O-O^{-} \leftrightarrow O^{-}-C=O-O^{2-} \leftrightarrow O^{-}-C=O-O^{-}} \end{array}$

Resonance Hybrid: The actual structure has all C–O bonds of equal length and strength, with the negative charge delocalized over all three oxygen atoms.

Phosphate Ion (PO43-)

The phosphate ion has several resonance structures, each with a double bond between phosphorus and a different oxygen atom. The negative charges are distributed among the oxygen atoms.

• Key Points:

  • Each resonance structure has one P=O double bond and three P–O single bonds.

  • The negative charges are located on the oxygen atoms with single bonds.

  • All four structures are equivalent.

Resonance Structures:

$\begin{array}{c} \mathrm{O^{-}-P=O-O^{-}-O^{-} \leftrightarrow O^{-}-P-O^{-}=O-O^{-} \leftrightarrow O^{-}-P-O^{-}-O^{-}=O \leftrightarrow O^{-}-P-O^{-}-O^{-}-O=O} \end{array}$

Resonance Hybrid: The actual structure has all P–O bonds of equal length and strength, with the negative charge delocalized over all four oxygen atoms.

Summary Table: Resonance Structures vs. Resonance Hybrid

Feature	Resonance Structures	Resonance Hybrid
Representation	Individual Lewis structures with electrons in different positions	Composite structure with delocalized electrons
Bond Lengths	Vary between single and double bonds	All bonds are equal (intermediate between single and double)
Charge Distribution	Localized on specific atoms	Delocalized over multiple atoms
Stability	Each structure is less stable	Hybrid is more stable

Additional info: Resonance is a key concept in GOB Chemistry for understanding the structure and reactivity of polyatomic ions and molecules. It helps explain why some bonds are of equal length and why certain ions are more stable than predicted by a single Lewis structure.