Draw a Lewis Structure for Nitric Acid (HNO3) Including All Three Resonance Structures

Nitric acid (HNO₃) is a strong acid and an important chemical in various applications. To draw its Lewis structure and the resonance structures, we start by determining the arrangement of electrons around the molecule.

The molecular structure consists of one nitrogen (N) atom bonded to three oxygen (O) atoms and one hydrogen (H) atom. The nitrogen atom forms one double bond with one of the oxygen atoms and single bonds with the other two oxygen atoms, one of which is also bonded to the hydrogen atom.

Step 1: Count valence electrons

First, we need to count the total number of valence electrons:

• N has 5 valence electrons.
• Each O has 6 valence electrons (3 x 6 = 18 for three O atoms).
• H has 1 valence electron.

Total = 5 + 18 + 1 = 24 valence electrons.

Step 2: Draw the basic structure

Start by placing the nitrogen atom in the center with single bonds to the three oxygen atoms and the hydrogen atom:

        O
        ||
    H - N - O
        |
        O

Step 3: Adding double bonds and lone pairs

Now, we will create the resonance structures by alternating the position of the double bond among the three oxygen atoms. Each time a double bond is formed, ensure the formal charge is calculated to see which resonance structure is the most stable.

Resonance Structures

1. O=N-O-H    (Oxygen with double bond is O, -1 formal charge)
   ||          (N has +1 formal charge)
   O          (O has 0 formal charge)

2. O-N=O-H    (Oxygen with double bond is N, 0 formal charge)
   ||          (N has 0 formal charge)
   O          (O has -1 formal charge)

3. O-N-O      (Oxygen with double bond is O, +1 formal charge on O)
      ||      (N has 0 formal charge)
      =O      (O has 0 formal charge)

Step 4: Formal Charges

To find the most stable resonance structure, we calculate formal charges:

• For the first structure (O=N-O-H): N has a +1 charge, one O has -1, the others are neutral.
• For the second structure (O-N=O-H): N has a 0 charge, O with the double bond has 0, and the other O has -1.
• For the third structure (O-N-O): here, one O has a +1 charge and exists with no negative charges.

The second resonance structure (O-N=O-H) is the most important because it has the least formal charge, leading to a more stable molecule. In practice, all three resonance forms contribute to the overall hybrid structure of nitric acid.