Question

Draw the Lewis structure for nitric acid (the hydrogen atom is attached to one of the oxygen atoms). Include all three resonance structures by alternating the double bond among the three oxygen atoms. Use formal charge to determine which of the resonance structures is most important to the structure of nitric acid.

Short answer

The most important resonance structure of nitric acid has the nitrogen atom double-bonded to one of the oxygen atoms (not the one bonded to hydrogen), with formal charges of 0 on the bonded oxygen and nitrogen, and +1 on the other oxygens.

Step-by-step solution

- Draw the Skeletal Structure

First, determine the skeletal structure of nitric acid (HNO3). The least electronegative atom, generally the central atom, is nitrogen (N). Oxygen atoms will be arranged around it. Hydrogen is always an outer atom and can only form one bond. Therefore, hydrogen is attached to one of the oxygen atoms.

- Distribute Electrons

Nitrogen has 5 valence electrons, oxygen has 6 valence electrons each, and hydrogen has 1 valence electron. Total valence electrons = 5 (N) + 3*6 (O) + 1 (H) = 24 electrons. Two electrons form a single bond between nitrogen and each oxygen, and one oxygen forms a single bond with hydrogen. The remaining electrons are distributed as lone pairs on the oxygen atoms.

- Complete Octets and Add Double Bonds

Ensure each atom (except hydrogen) has a complete octet, redistributing electrons as needed. After placing initial lone pairs, create a double bond between nitrogen and one of the oxygens to allow nitrogen to achieve an octet.

- Draw Resonance Structures

Draw the three resonance structures by moving the location of the double bond among the three oxygen atoms while keeping the hydrogen attached to one particular oxygen.

- Calculate Formal Charges

For each resonance structure, calculate the formal charge on each atom using the formula: Formal charge = (valence electrons) - (lone electrons + 1/2 bonded electrons). Adjust the resonance structures based on lower formal charges where possible.

- Determine the Most Important Resonance Structure

The most important resonance structure has the lowest energy, indicated by the formal charges being as close to zero as possible for each atom, obeying the octet rule, and placing any negative charges on the more electronegative atoms (oxygen in this case).

Key concepts

Resonance Structures

Resonance structures are different ways of drawing the same molecule that illustrate the delocalization of electrons within the molecule. In the case of nitric acid (HNO3), resonance is used to show how the double bond can be placed between the nitrogen atom and any of the three oxygen atoms.

When drawing the resonance structures, it's important to remember that only the electron placement changes, not the actual positions of the atoms. For nitric acid, we have three resonance structures essentially because of the three oxygen atoms that can each take turns forming a double bond with the nitrogen atom. The presence of these resonance structures indicates that the real structure is a hybrid of all the possible structures.

Visualizing Electron Delocalization

By visualizing the different resonance forms, we get a sense of how the π (pi) electrons are delocalized across the molecule. This delocalization leads to increased stability because the electrons are more spread out rather than being concentrated in one region. Resonance structures are essentially a way to represent molecule stability that is not evident from a single structural formula.

Formal Charge

The concept of formal charge plays a crucial role in determining the most stable resonance structure of a molecule. Formal charge is an accounting tool calculated from the Lewis structure and provides insight into the electron distribution within a molecule.

To calculate the formal charge for each atom, we use the formula:
\[\begin{equation}\text{Formal charge} = (\text{valence electrons}) - (\text{lone electrons} + \frac{1}{2}\text{bonded electrons}).\end{equation}\]
The computed formal charges help chemists determine the best representation of a molecule's structure. The most stable resonance structure is typically the one where the formal charges are minimized, with negative formal charges residing on more electronegative atoms, and positive formal charges, if any, on less electronegative atoms.

Importance in Resonance

Comparing the formal charges on the different possible resonance structures of nitric acid tells us not only which structure contributes most to the hybrid form but also how electrons are likely to be distributed in the actual molecule. Thus, the concept of formal charge directly affects our understanding of molecular stability and electron distribution.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are critical to understanding chemical bonding and molecule structure. They are the ones involved in forming bonds with other atoms. Nitrogen, for instance, has 5 valence electrons, while each oxygen has 6 and hydrogen has 1.

In the case of nitric acid, the sum of all valence electrons from nitrogen, hydrogen, and the three oxygen atoms gives us a total that we use to fill the atomic