Lewis structure of SO₃
24 valence electrons. Three S=O double bonds, with no lone pair on S, so S has 12 electrons around it (an expanded octet).
Lewis structure
24 valence electronsi
Drawn with three S=O bonds, an expanded octet with no formal charges, the way NCERT draws H2SO4.
Formal charges: none
Why this shape
- S has 6 valence electrons. It uses six in three double bonds to O, so none are left over.
- Steric number = three sigma bonds + no lone pairs = 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 no lone pairs the atoms take the same shape: trigonal planar.
- All the bond pairs repel each other equally, so the angle is the ideal 120°.
- The bond dipoles cancel in the symmetric trigonal planar shape, so SO₃ is non-polar.
How to draw it
- Count the valence electrons: S 6 + 3 × O 6 = 24 valence electrons, 12 pairs.
- Put S in the middle and join it to the three 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: three S=O double bonds, with no lone pair on S, so S has 12 electrons around it (an expanded octet).
- Formal charge = valence electrons − lone pair electrons − number of bonds. Here: none.
Class 11, Chapter 4 Chemical Bonding and Molecular Structure.
Lewis structure of SO3: quick answers
How many valence electrons are in SO3?
S 6 + 3 × O 6 = 24 valence electrons, 12 pairs.
What are the formal charges in the Lewis structure of SO3?
Every atom has a formal charge of zero.
How many lone pairs are in the Lewis structure of SO3?
6 in all, none on the central S.