What is the Lewis structure of nitrous acid (HNO₂) and what are the electron pair and molecular geometries of the internal oxygen and nitrogen atoms?

Answer:

The Lewis structure of nitrous acid (HNO₂) can be drawn by considering the following steps:

1. Count the total number of valence electrons: Nitrogen has 5, each oxygen has 6, and hydrogen has 1. So, 5 + 6 + 6 + 1 = 18 valence electrons.
2. Place the nitrogen atom in the center as it is the least electronegative. Attach one hydrogen atom and two oxygen atoms to it.
3. Distribute the remaining electrons to ensure that each atom has an octet (or full valence shell). The final structure shows nitrogen double-bonded to one oxygen and single-bonded to the other oxygen, which is in turn bonded to hydrogen.

The resulting Lewis structure can be represented as:

Next, let’s examine the geometries:

For the internal atoms:

• The nitrogen atom has three regions of electron density (one double bond with oxygen and one single bond with oxygen). Therefore, the electron pair geometry is trigonal planar, and the molecular geometry is also trigonal planar.
• The internal oxygen atom (double-bonded to nitrogen) has two regions of electron density (one double bond with nitrogen and two lone pairs). Hence, its electron pair geometry is bent (or angular), and its molecular geometry is also bent.

Now, regarding hybridization:

• The internal nitrogen atom undergoes sp² hybridization due to its trigonal planar arrangement of electron pairs.
• The internal oxygen atom, which has a bent molecular shape, exhibits sp³ hybridization as it has one double bond and two lone pairs.

In summary:

• Lewis structure: as shown above.
• Nitrogen: electron pair geometry is trigonal planar; hybridization is sp².
• Internal oxygen (double-bonded): electron pair geometry is bent; hybridization is sp³.