Write the Lewis structures for the following molecules, including all resonance forms, and show formal charges

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Write the Lewis structures for the following molecules, including all resonance forms, and show formal charges

1) HCO2-

2) CH2NO2-

3) . CH2N2

The correct answer and explanation is:

1. HCO₂⁻ (Formate Ion)

The HCO₂⁻ ion has a structure where the central carbon atom is bonded to a hydrogen atom and two oxygen atoms. The ion has a negative charge, which is typically placed on one of the oxygen atoms, as oxygen is more electronegative and can better accommodate the negative charge.

Lewis Structure:

  • The carbon atom is single-bonded to a hydrogen atom and to two oxygen atoms.
  • One oxygen is double-bonded to carbon, while the other oxygen is single-bonded and carries the negative charge.
  • The formal charges are as follows:
    • The carbon atom has a formal charge of 0.
    • One oxygen has a formal charge of 0 (double-bonded to carbon).
    • The second oxygen (with the negative charge) has a formal charge of -1.
    • The hydrogen atom has a formal charge of 0.

Resonance Forms:

The negative charge on oxygen can be delocalized, creating resonance between the two oxygen atoms. The two resonance structures are as follows:

  1. Oxygen double-bonded to carbon, and the second oxygen carries the negative charge.
  2. The negative charge moves to the oxygen that was double-bonded, and the other oxygen becomes single-bonded.

2. CH₂NO₂⁻ (Nitromethane Ion)

The CH₂NO₂⁻ ion consists of a carbon atom bonded to two hydrogen atoms and a nitro group (NO₂) with a negative charge.

Lewis Structure:

  • The carbon atom is bonded to two hydrogen atoms and one nitrogen atom, which is further bonded to two oxygens (one double-bonded, the other single-bonded).
  • The negative charge resides on one of the oxygen atoms in the nitro group.
  • The formal charges are:
    • Carbon has a formal charge of 0.
    • The nitrogen has a formal charge of 0.
    • One oxygen (double-bonded) has a formal charge of 0.
    • The other oxygen (single-bonded and carrying the negative charge) has a formal charge of -1.
    • The two hydrogens have formal charges of 0.

Resonance Forms:

The negative charge can shift between the two oxygens of the nitro group, creating two resonance structures. In each resonance form, the negative charge alternates between the oxygens.

3. CH₂N₂ (Diazomethane)

CH₂N₂ is a molecule with a central carbon atom bonded to two hydrogens and a diazo group (N₂).

Lewis Structure:

  • The central carbon atom is single-bonded to two hydrogens and double-bonded to one nitrogen atom.
  • The second nitrogen atom is bonded to the first nitrogen atom with a single bond.
  • The formal charges are:
    • Carbon has a formal charge of 0.
    • Each nitrogen atom carries a formal charge of 0.
    • The hydrogens have formal charges of 0.

Resonance Forms:

Due to the presence of the N₂ group, there are no significant resonance structures in CH₂N₂, as the bond between the nitrogen atoms is a single bond.

General Concept of Formal Charges:

Formal charges can be calculated by the formula:
Formal Charge = (Valence electrons of atom) – (Non-bonding electrons) – (Bonding electrons/2)
The goal is to minimize formal charges, with the negative charge generally residing on the more electronegative atoms (like oxygen and nitrogen).

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