Lewis structure

Lewis structure of SO₂

18 valence electrons. One S=O double bond and one S-O single bond, with one lone pair on S. 2 resonance structures.

Molecule or ion

SO₂Sulphur dioxide

Bent

AB₂E: 2 bond pairs, 1 lone pair

18 valence electrons, 2 resonance structures

i

Two resonance structures with octets. It is also often drawn as O=S=O with no charges (expanded octet).

Formal charges: S +1, O −1

S has 6 valence electrons. In the Lewis structure it makes one double bond to O and one single bond to O and keeps one lone pair, which leaves it a formal charge of +1.
Steric number = two sigma bonds + one lone pair = 3, so S is sp² hybridised. A double or triple bond counts once: only its sigma bond uses a hybrid orbital.
Three electron pairs point to the corners of a flat triangle (trigonal planar electron geometry). With one lone pair in place of atoms, the atoms make a bent shape.
Repulsion falls in the order lone pair to lone pair > lone pair to bond pair > bond pair to bond pair. The lone pair pushes the bond pairs closer, so the angle is about 119°, less than the ideal 120°.
The bond dipoles do not cancel in the bent shape, so SO₂ is polar.

Working

How to draw it

Count the valence electrons: S 6 + 2 × O 6 = 18 valence electrons, 9 pairs.
Put S in the middle and join it to the two outer atoms with single bonds.
Give each outer atom lone pairs until it has an octet (H needs only two electrons), then put any electrons left over on the central atom.
Where an atom is still short of an octet, turn a lone pair of its neighbour into a shared pair: that makes a double or triple bond.
Result: one S=O double bond and one S-O single bond, with one lone pair on S.
Formal charge = valence electrons − lone pair electrons − number of bonds. Here: S +1, O −1.
The double bond can sit in two equivalent ways, so there are two resonance structures. The real molecule is a hybrid of them.

Class 11, Chapter 4 Chemical Bonding and Molecular Structure.

Questions

How many valence electrons are in SO2?

S 6 + 2 × O 6 = 18 valence electrons, 9 pairs.

What are the formal charges in the Lewis structure of SO2?

S +1, O −1; every other atom is zero. Formal charge = valence electrons - lone pair electrons - number of bonds.

How many resonance structures does SO2 have?

2. The double bond can sit in two equivalent ways, so there are two resonance structures. The real molecule is a hybrid of them.