To draw the Lewis structure for SO₃²⁻ (sulfite ion), follow these steps:
- Count the total number of valence electrons:
- Sulfur (S) has 6 valence electrons.
- Each oxygen (O) has 6 valence electrons, and there are 3 oxygen atoms, so 3 × 6 = 18 electrons.
- The ion has a 2- charge, which means there are 2 additional electrons.
- Total valence electrons = 6 (S) + 18 (O) + 2 (charge) = 26 electrons.
- Place the least electronegative atom in the center:
- Sulfur is less electronegative than oxygen, so it will be the central atom.
- Connect the central atom to the surrounding atoms with single bonds:
- Draw single bonds between sulfur and each oxygen atom. This uses 6 electrons (3 bonds × 2 electrons).
- Distribute the remaining electrons:
- Subtract the electrons used in bonding from the total: 26 – 6 = 20 electrons remaining.
- Place lone pairs on the oxygen atoms first. Each oxygen needs 6 electrons to complete its octet (2 electrons are already from the bond with sulfur).
- Each oxygen will have 3 lone pairs (6 electrons). This uses 18 electrons (3 oxygens × 6 electrons).
- Remaining electrons: 20 – 18 = 2 electrons. Place these as a lone pair on the sulfur atom.
- Check for octet rule:
- Sulfur has 8 electrons around it (2 from the lone pair and 6 from the bonds), satisfying the octet rule.
- Each oxygen also has 8 electrons (6 from lone pairs and 2 from the bond with sulfur).
Number of Bonds and Non-Bonding Pairs:
- Sulfur forms 3 single bonds with oxygen atoms.
- Sulfur has 1 lone pair of electrons.
Shape of the Molecule:
- The sulfite ion (SO₃²⁻) has a trigonal pyramidal shape.
- This is because there are 3 bonding pairs and 1 lone pair around the central sulfur atom, leading to a tetrahedral electron pair geometry but a trigonal pyramidal molecular geometry due to the lone pair.