Draw the best Lewis dot structure for ozone, O3, complete with resonance structures. What is the formal charge on the central oxygen? Note that ozone is V-shaped. It is not a closed equilateral triangle.
A +1
B 0
C -1
D +2
E -2
The correct answer and explanation is :
To draw the best Lewis dot structure for ozone (O₃) and determine the formal charge on the central oxygen, let’s go through the process step by step.
1. Electron Counting:
Ozone (O₃) consists of three oxygen atoms, each contributing 6 valence electrons, giving a total of 18 valence electrons (3 × 6 = 18). These electrons will be used to form bonds and lone pairs.
2. Initial Bonding:
In ozone, the central oxygen atom is bonded to two other oxygen atoms. Start by connecting the oxygen atoms with single bonds. This consumes 2 electrons (1 bond = 2 electrons), and we have 16 electrons left to distribute as lone pairs.
3. Distribute Remaining Electrons:
After forming the single bonds, place the remaining electrons as lone pairs around each oxygen atom. Initially, give each oxygen atom (except the central one) 3 lone pairs (6 electrons). After placing the lone pairs, we have 4 electrons remaining.
4. Double Bond Formation:
To minimize formal charges, convert one of the lone pairs on an outer oxygen atom into a double bond with the central oxygen atom. This reduces the formal charges and satisfies the octet rule for each oxygen atom.
5. Resonance Structures:
Ozone has resonance structures because the double bond between the central and one of the outer oxygen atoms can be swapped with the other outer oxygen. This means there are two possible resonance structures for ozone:
- One structure has a double bond between the central oxygen and one outer oxygen, with a single bond to the other outer oxygen.
- The other resonance structure has the double bond with the other outer oxygen and a single bond to the first oxygen.
6. Formal Charges:
Now, let’s calculate the formal charge on each atom. The formula for formal charge is:
[
\text{Formal charge} = \text{Valence electrons} – (\text{Nonbonding electrons} + \frac{\text{Bonding electrons}}{2})
]
- For the central oxygen:
- Valence electrons = 6
- Nonbonding electrons = 2 (one lone pair)
- Bonding electrons = 4 (from two bonds, one double and one single)
- Formal charge = 6 – (2 + 4/2) = 6 – 4 = +1
- For the outer oxygen with a single bond:
- Valence electrons = 6
- Nonbonding electrons = 6
- Bonding electrons = 2 (one single bond)
- Formal charge = 6 – (6 + 2/2) = 6 – 7 = -1
- For the outer oxygen with a double bond:
- Valence electrons = 6
- Nonbonding electrons = 4
- Bonding electrons = 4 (one double bond)
- Formal charge = 6 – (4 + 4/2) = 6 – 6 = 0
7. Conclusion:
The formal charge on the central oxygen atom is +1. This is the best answer, as it minimizes formal charges and satisfies the octet rule. The correct answer is A: +1.
Explanation of the Structure:
The V-shape of the ozone molecule results from the fact that it has a bent geometry due to the lone pairs on the oxygen atoms. This structure, with resonance, reflects the true bonding arrangement, where the central oxygen carries a +1 formal charge in order to balance the formal charges on the outer oxygens. The overall molecule is neutral, as the negative formal charges on the outer oxygens balance the positive formal charge on the central oxygen.