What is the formal charge on the sulfur atom in SO3
The Correct Answer and Explanation is:
Correct Answer:
The formal charge on the sulfur atom in SO₃ (sulfur trioxide) is 0.
Explanation
To determine the formal charge on the sulfur atom in sulfur trioxide (SO₃), it is necessary to use the formal charge formula:
Formal Charge = (Valence electrons) – (Nonbonding electrons) – (Bonding electrons ÷ 2)
Sulfur, located in group 16 of the periodic table, has 6 valence electrons. In the SO₃ molecule, sulfur forms three double bonds with three oxygen atoms, resulting in six bonding pairs or 12 bonding electrons. Since all bonding electrons are shared, sulfur receives half of the bonding electrons, which is 6 electrons.
Additionally, sulfur in SO₃ has no lone pairs (nonbonding electrons). This is because all of its valence electrons are involved in bonding with oxygen atoms.
Plugging into the formula:
- Valence electrons of sulfur = 6
- Nonbonding electrons = 0
- Bonding electrons = 12 → 12 ÷ 2 = 6
Formal charge on sulfur = 6 – 0 – 6 = 0
Thus, the formal charge on the sulfur atom in SO₃ is zero.
Sulfur can exceed the octet rule because it is in the third period of the periodic table, which allows access to d-orbitals. This makes it possible for sulfur to form more than four bonds. In SO₃, this expanded octet structure allows sulfur to form three double bonds, which distributes electron density evenly and minimizes formal charges on all atoms in the molecule.
Each oxygen atom also ends up with a formal charge of zero in this structure. This arrangement is considered the most stable resonance form of SO₃. The formal charge calculation plays a crucial role in predicting and confirming the most stable Lewis structure of a molecule.
