To draw the Lewis structure for SO₃²⁻ (sulfite ion) and determine its properties, follow these steps:
Step 1: Count the Total Number of Valence Electrons
Sulfur (S) has 6 valence electrons, and each oxygen (O) has 6 valence electrons. Since there are 3 oxygen atoms, the total number of valence electrons is:
6 (S) + 3 × 6 (O) + 2 (for the 2- charge) = 26 valence electrons.
Step 2: Draw the Skeleton Structure
Place the sulfur atom in the center and connect it to the three oxygen atoms with single bonds. This uses 6 electrons (3 bonds × 2 electrons each).
Step 3: Distribute the Remaining Electrons
Subtract the electrons used in the bonds from the total valence electrons: 26 – 6 = 20 electrons. Distribute these electrons as lone pairs around the oxygen atoms to satisfy the octet rule. Each oxygen atom should have 8 electrons (2 in the bond and 6 as lone pairs).
Step 4: Check for Octet Rule
After distributing the electrons, sulfur will have 12 electrons around it, which is more than the octet rule allows. To fix this, move one lone pair from an oxygen atom to form a double bond with sulfur. This will reduce the number of electrons around sulfur to 8.
Step 5: Determine the Electron Geometry
The electron geometry is determined by the arrangement of electron pairs around the central atom. In SO₃²⁻, there are 3 bonding pairs and 1 lone pair around sulfur, giving it a tetrahedral electron geometry.
Step 6: Determine the Molecular Geometry
The molecular geometry is determined by the arrangement of atoms around the central atom, ignoring lone pairs. In SO₃²⁻, the molecular geometry is trigonal pyramidal due to the lone pair on sulfur.
Step 7: Determine the Hybridization
The hybridization of the central atom (sulfur) can be determined by counting the number of electron domains (bonding pairs and lone pairs). In SO₃²⁻, sulfur has 4 electron domains (3 bonding pairs and 1 lone pair), so it is sp³ hybridized.
Step 8: Determine the Polarity
To determine the polarity of SO₃²⁻, consider the molecular geometry and the electronegativity of the atoms. The sulfite ion has a trigonal pyramidal shape, and the electronegativity difference between sulfur and oxygen creates polar bonds. However, due to the symmetrical arrangement of the oxygen atoms, the dipole moments cancel out, making the molecule nonpolar overall.
In summary, the Lewis structure for SO₃²⁻ shows a trigonal pyramidal molecular geometry, sp³ hybridization, and it is nonpolar.