Problem 53 - Chemistry The Central Science In Si Units

Question

(a) Draw the best Lewis structure(s) 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?

Step-by-Step Solution

Answer

(a) Draw resonance structures for \(\mathrm{NO}_{2}^{-}\). (b) It's isoelectronic with \(\mathrm{O}_{3}\). (c) Bond lengths are between those of N-O single and double bonds.

Step 1: Calculate Total Valence Electrons

Firstly, determine the total number of valence electrons available in the nitrite ion \(\mathrm{NO}_{2}^{-}\). Nitrogen has 5 valence electrons, each oxygen has 6, adding up to 12 valence electrons for the two oxygens. The \(\mathrm{NO}_{2}^{-}\) ion carries a negative charge, providing one additional electron. Therefore, the total is \(5 + 12 + 1 = 18\) valence electrons.

Step 2: Draw the Skeletal Structure

Draw the basic structure for \(\mathrm{NO}_{2}^{-}\) with nitrogen as the central atom bonded to two oxygen atoms. Place single bonds between the nitrogen and each oxygen initially.

Step 3: Distribute Remaining Electrons

After establishing the skeletal structure with single bonds, use the remaining 14 valence electrons (after accounting for 4 used bonding electrons in single bonds) to complete the octets for the oxygen atoms. Place 6 electrons (3 lone pairs) around each oxygen atom.

Step 4: Create Double Bonds to Minimize Formal Charges

To minimize formal charges, create a double bond between the nitrogen and one oxygen atom. This will result in resonance structures: one structure with a double bond to the left oxygen and another with a double bond to the right oxygen. The structure alternates between \([\mathrm{N} - \mathrm{O}]^{1-}\) and \([\mathrm{N} = \mathrm{O}]^{0}\).

Step 5: Assign Formal Charges

Calculate the formal charges: for nitrogen, it will be 0 as \((5 - (0 + 4)) = 0\), for the double-bonded oxygen, it will be 0, and for the single-bonded oxygen, formal charge will be -1. The resonance structures balance these charges with an overall -1 charge on the ion.

Step 6: Determine Isoelectronic Allotrope

Identify the isoelectronic allotrope of oxygen. \(\mathrm{NO}_{2}^{-}\) has the same number of electrons (18) as ozone \(\mathrm{O}_{3}\). Thus, \(\mathrm{NO}_{2}^{-}\) is isoelectronic with \(\mathrm{O}_{3}\).

Step 7: Predict Bond Lengths

In \(\mathrm{NO}_{2}^{-}\), the bond lengths are intermediate between a single bond \((\sim 143\ \mathrm{pm})\) and a double bond \((\sim121\ \mathrm{pm})\). Due to resonance, both N-O bonds are about 123 pm, indicating bond order is between 1 and 2.

Key Concepts

Resonance Structures

When we draw Lewis structures for molecules like No2^-, we often find multiple valid structures that illustrate how electrons can be distributed.
These are called resonance structures and represent different possible distributions of electrons across bonds in the molecule. ### Importance of Resonance - **Electron Distribution:** Resonance structures display the possible locations of electron pairs in molecules. This helps in understanding the molecule's reactivity and properties. - **Stability:** Resonance increases stability because the actual distribution of electrons is a blend of its possible resonance forms. ### Resonance in No2^- - **Skeletal Structure:** Initially, we place single bonds between nitrogen and the two oxygen atoms. - **Electron Arrangement:** Electrons are then arranged to ensure the octet rule is satisfied.
After placing single bonds, resonance structures are developed by considering alternate double bonds between nitrogen and either of the oxygen atoms. - **Visualizing Resonance:** The structure shows a double bond with one oxygen and a single bond with the second oxygen; then alternates, showing the double bond with the other oxygen. ### Consequence of Resonance In the nitrite ion, resonance implies that neither of the nitrogen-oxygen bonds is a complete single or double bond. Instead, they are a hybrid, resulting in molecular stability and intermediate bond lengths.

Formal Charge

Formal charge is a concept used to estimate the charge distribution within a molecule.
It helps chemists understand which lewis structure is the most appropriate depiction or has the lowest energy state.### Calculating Formal ChargeThe formal charge on an atom can be calculated with the formula:\[ \text{Formal Charge} = \text{Valence Electrons} - (\text{Non-bonded Electrons} + \frac{\text{Bonded Electrons}}{2}) \]### Application in No2^-- **Nitrogen:** Has 5 valence electrons, 0 non-bonded electrons, and shares 4 from bonds, leading to zero formal charge.- **Double-bonded Oxygen:** Possesses 6 valence electrons, 4 non-bonded electrons, and 4 electrons in bonds. This results in a formal charge of zero.- **Single-bonded Oxygen:** With the same method, its formal charge becomes -1 because it has excess electrons compared to the neutral state.### Significance- **Identifying the Best Structure:** The structure where formal charges are closest to zero is typically considered the most stable.- **Balancing Charge:** It's essential for the overall molecule, such as No2^-,to reflect the ion's total charge, which is -1.

Isoelectronic Species

Species that have the same number of electrons and a similar electronic structure are known as isoelectronic.
This concept helps us identify and predict properties and behaviors of different ions and molecules. ### Understanding Isoelectronic Species - **Similar Electron Counts:** Isoelectronic species share identical total electron numbers. - **Comparative Analysis:** Observing properties shared across different molecules or ions enables predictive and comparative analysis. ### Isoelectronic Pairing In the exercise, No2^- is revealed to be isoelectronic with ozone O_3. - **Electron Count:** Both contain 18 electrons. - **Similar Structural Features:** Despite differing elements, structural electron distribution can show similarities. ### How It Helps - **Predicting Properties:** Knowing two species are isoelectronic provides insights into potential chemical behaviors and properties. - **Exploring Reactivity:** Isoelectronic molecules may have similar reactivity or polarity attributes, giving clues about molecular interactions.
This is beneficial when designing reactions or understanding molecular structures in chemistry.