Draw the electron dot formula for sulfur trioxide; SO3. How many pairs of nonbonding electrons are in the sulfur trioxide molecule?
The correct answer and explanation is:
Electron Dot Formula of Sulfur Trioxide (SO₃):
The Lewis structure of sulfur trioxide (SO₃) consists of a sulfur atom at the center, bonded to three oxygen atoms through double bonds. There are no lone pairs of electrons on the central sulfur atom, but each oxygen atom has two lone pairs (nonbonding pairs) of electrons.
Lewis Structure:
O
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O == S == O
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O
Each oxygen atom forms a double bond with sulfur and retains two lone pairs of electrons. The sulfur atom uses twelve electrons (six bonds) around it, which is an exception to the octet rule.
Number of Nonbonding Electron Pairs:
Each oxygen atom has 2 lone pairs. There are 3 oxygen atoms.
- 2 lone pairs per oxygen × 3 oxygens = 6 lone pairs
So, there are 6 pairs of nonbonding electrons in the SO₃ molecule.
Explanation (300 words):
Sulfur trioxide (SO₃) is a covalent compound composed of one sulfur atom and three oxygen atoms. To draw its electron dot or Lewis structure, we must follow a systematic approach based on valence electrons.
Sulfur (S) has 6 valence electrons, and each oxygen (O) also has 6 valence electrons. Therefore, the total number of valence electrons for SO₃ is:
- 1 sulfur × 6 electrons = 6 electrons
- 3 oxygen × 6 electrons = 18 electrons
- Total = 24 valence electrons
To satisfy the bonding requirements and minimize formal charges, each oxygen forms a double bond with the sulfur atom. This uses 6 bonding pairs (12 electrons). The remaining 12 electrons are distributed as lone pairs on the oxygen atoms, 4 electrons (2 lone pairs) per oxygen.
The sulfur atom ends up forming three double bonds and has no lone pairs. Although this exceeds the octet rule, sulfur can do so because it is in period 3 and has access to empty d-orbitals, allowing expansion of its valence shell.
Each oxygen atom has two nonbonding pairs. Since there are three oxygen atoms:
- 2 pairs × 3 = 6 nonbonding electron pairs
This structure provides complete octets for each oxygen and an expanded octet for sulfur. It is the most stable and commonly accepted Lewis structure for SO₃. The molecule is planar and symmetrical, making it nonpolar despite having polar bonds.