Question

Draw reasonable resonance structures for the following ions: (a) \(\mathrm{HSO}_{4}^{-},\) (b) \(\mathrm{PO}_{4}^{3-}\), (c) \(\mathrm{HPO}_{3}^{2-},(\mathrm{d}) \mathrm{IO}_{3}^{-}\)

Short answer

Resonance structures are created by redistributing electrons as alternative Lewis structures for each ion.

Step-by-step solution

Identify the Central Atom

For each ion, identify the central atom which will be bonded to the other atoms. Generally, the least electronegative atom is the central atom. In (a) \(\mathrm{HSO}_{4}^{-}\), sulfur is the central atom. In (b) \(\mathrm{PO}_{4}^{3-}\), phosphorus is the central atom. In (c) \(\mathrm{HPO}_{3}^{2-}\), phosphorus is again the central atom. In (d) \(\mathrm{IO}_{3}^{-}\), iodine is the central atom.

Determine Initial Lewis Structure

Draw the initial Lewis structure for each ion, making sure to account for all valence electrons. Include any formal charges on atoms by ensuring that the total charge of the ion is correct. Begin with single bonds around the central atom and distribute electrons to satisfy the octet rule where applicable.

Create Resonance Structures

For each ion, adjust the positions of double bonds and lone pairs to create alternative reasonable resonance structures. This involves shifting electrons to form double bonds between different atoms and distribute formal charges as evenly as possible.

Verify Resonance Stability

Ensure that each resonance structure respects the octet rule and minimizes charge separation where possible, resulting in more stable structures. Typically, resonance structures with less charge separation and complete octet around each atom (where applicable) are more stable.

Resonate and Draw

Illustrate the resonance structures by showing arrows indicating the movement of electrons between structures. For each of the ions, draw at least two resonance structures if possible, and represent delocalization with double-headed arrows indicating resonance.

Key concepts

Lewis structure

A Lewis structure is a visual representation of the arrangement of atoms within a molecule, showing how the valence electrons are distributed among them. It helps in visualizing how atoms bond together and can predict the shape and reactivity of the molecule. When drawing a Lewis structure:

• Identify the total number of valence electrons available. This includes the electrons for each atom in the ion and any additional electrons from negative charges or fewer electrons from positive charges.
• Arrange the atoms to establish the connectivity. Typically, the least electronegative atom is chosen as the central atom, around which other atoms are arranged.
• Place single bonds between the central atom and surrounding atoms, using two electrons for each bond.
• Distribute the remaining electrons to complete the octet for each atom, beginning with the outer atoms and moving inward as needed.

For ions, it is crucial to ensure that the final structure corresponds to the correct overall charge. Add or subtract electrons to achieve this, while minimizing formal charges.

formal charges

Formal charges are a bookkeeping tool used in chemistry to indicate the distribution of electric charge in a molecule based on its Lewis structure. Calculating formal charges is crucial for identifying the most stable resonance structures. The formula for calculating the formal charge on an atom in a molecule is:\[ \text{Formal Charge} = (\text{Number of Valence Electrons in Free Atom}) - (\text{Number of Non-Bonding Electrons}) - \frac{1}{2}(\text{Number of Bonding Electrons}) \]

• Use this formula to assign formal charges to each atom in the resonance structure. This helps identify atoms that carry more (or less) charge than their neutral state.
• Preferred resonance structures have the smallest possible formal charges across the atoms, especially for the central atom.
• Avoid placing negative formal charges on highly electronegative atoms, as these are typically less stable.

By calculating and assigning formal charges, one can evaluate the viability and stability of different resonance forms for a given ionic species.

octet rule

The octet rule is a key principle in chemistry that states atoms tend to form bonds until they are surrounded by eight electrons. This mimics the electron configuration of a noble gas, which is often associated with stability. While drawing resonance structures, the octet rule guides the distribution of electrons:

• Each atom should ideally have eight electrons around it after bonds and lone pairs are accounted for. The major players like carbon, nitrogen, oxygen, and halogens generally follow this rule.
• Note that some elements, especially nonmetals in the third period or higher (such as sulfur or phosphorus), may have expanded octets due to available d-orbitals, allowing them to hold more than eight electrons.
• Observing the octet rule allows for stable drafting of Lewis and resonance structures, underlining the importance of electron distribution for chemical stability.

It's crucial to remember that while octet completion is typical, there are exceptions, especially in molecules or ions with an odd number of electrons or elements that naturally possess less than an octet in stable forms.

electron distribution

Electron distribution in molecules is vital for understanding the chemical behavior and properties of a substance. Proper electron distribution ensures that the stability and reactivity of the molecule are maintained. The process involves the following:

• Electrons are first assigned to form bonds, especially with the central atom. Ensure that enough electrons help form the requisite number of bonds corresponding to the determined connectivity of atoms.
• Upon distributing bonding electrons, place any remaining electrons as lone pairs on the atoms, preferably starting from outer atoms and proceeding inward, sticking to the octet rule where applicable.
• During resonance structure formation, electron distribution may change as double bonds or lone pairs shift positions. This does not change the number of electrons but rather their placement.

Accurate electron distribution helps in understanding molecular dipoles, potential reactivity sites, and interaction with other molecules, playing a critical role in predicting the behavior of ionic compounds like the ones discussed in the exercise.