SO2 Lewis Structure
Sulfur dioxide · A sharp-smelling gas released by burning sulfur-containing fuels and by volcanoes.
The SO2 Lewis structure has 18 valence electrons, drawn as one S–O bond and one S=O bond, with six lone pairs in total. The central S is sp2 hybridized, which makes it a bent molecule with a bond angle of 119 degrees. SO2 is polar: the S-O bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel. There are two equivalent resonance structures.
- 18 valence e−
- bent
- 119° bond angle
- sp2
- polar
Calculated properties
| Total valence electrons | 18 |
|---|---|
| Bonding electrons | 6 (3 shared pairs) |
| Nonbonding electrons | 12 (6 lone pairs) |
| Lone pairs on S | 1 |
| Electron domains (steric number) | 3 |
| VSEPR notation | AX2E1 |
| Electron geometry | trigonal planar |
| Molecular geometry | bent |
| Bond angle | 119° |
| Hybridization | sp2 |
| Polarity | polar |
| Formal charges | S: +1, O: -1 |
| Resonance structures | 2 |
| Molar mass | 64.058 g/mol |
How to draw the SO2 Lewis structure
Step through it. The bar shows how much of the 18-electron budget is spent at each stage - it never grows, which is the whole constraint.
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Skeleton. Join every connected pair with one bond. 2 bonds spends 4 electrons.
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Outer octets. Spend electrons as lone pairs on the outer atoms until each has eight. 2 are still unplaced.
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Centre. Everything still unplaced goes on S, which now has 6 electrons around it.
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Multiple bonds. The central atom is still short of an octet, so one lone pair on the outer atoms moves in to be shared. The electron count does not change - the same electrons are just counted by both atoms now.
The reasoning at each step
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Count the valence electrons
Add up the outer-shell electrons every atom brings. That total is the budget for the whole structure - every line and every dot has to come out of it, and nothing may be added.
6 (S) + 2 x 6 (O) = 18 valence electrons -
Choose the central atom
S is the least electronegative atom here, so it takes the centre. Hydrogen is never central: it can only form one bond.
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Join everything with single bonds first
Every connection starts as one shared pair. Draw them all before worrying about double bonds - the arithmetic in the next step is what tells you where the double bonds have to go.
2 bonds x 2 = 4 electrons used, 14 left -
Work out how far short you are
Add up what every atom still needs to fill its shell - eight electrons for most atoms, two for hydrogen - minus what its single bonds already give it. Compare that with the electrons you have left. The difference decides everything that follows.
needs 16, has 14, short by 2 -
Turn the shortfall into multiple bonds
Being short means atoms have to share more. Every pair that moves from a lone pair into a bond counts twice - once for each atom - so a shortfall of 2 is covered by 1 extra shared pair. That is where the double bond comes from.
S=O2 -
Fill in the lone pairs
Every electron not in a bond sits as a lone pair on an atom. Each atom takes exactly what it needs to finish its shell - there is no choice left at this point.
S: 1, O1: 3, O2: 2 (12 electrons) -
Check the central atom
S ends up with a full octet. Every electron in the budget is now placed.
S: 6 bonding + 2 nonbonding = 8 -
Check the formal charges
Formal charge is valence electrons, minus lone-pair electrons, minus the number of bonds. They are not real charges, but they have to add up to the overall charge on the species, and a structure that keeps them small is the better one.
S: 6 - 2 - 3 = +1 ; O1: 6 - 6 - 1 = -1 -
Work out the shape
Count the electron domains on S: 2 bonded groups plus 1 lone pair. A double or triple bond still counts as one domain, because it points in one direction. That gives trigonal planar electron geometry; ignore the lone pairs and the atoms themselves sit in a bent arrangement. The lone pair takes up more room than a bonding pair, squeezing the bond angle just under the ideal 120 degrees.
steric number 3 -> sp2 -> bent, bond angle 119 -
Decide whether it is polar
The S-O bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.
dipoles do not cancel -> polar
Resonance structures
More than one drawing gives the same electron count and the same formal charges, and no single one of them is the real molecule. SO2 is an average of the 2 structures below, which is why bonds that look different here are actually identical in the real molecule.
The expanded-octet version
Courses disagree about SO2. The structure at the top of this page obeys the octet rule and leaves formal charges behind. The one below puts more than eight electrons on the central atom and brings every formal charge to zero. Check which convention your course uses.
Is SO2 polar or nonpolar?
SO2 is polar. The S-O bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.
| Bond | Electronegativity difference | Character |
|---|---|---|
| S–O | 0.86 | polar covalent |
Common questions
How many valence electrons does SO2 have?
SO2 has 18 valence electrons. 6 of them are in bonds and 12 sit in lone pairs.
What is the molecular geometry of SO2?
SO2 is a bent molecule. The central S has 2 bonded groups and one lone pair, a steric number of 3, which gives trigonal planar electron geometry and a bent molecule.
What is the bond angle in SO2?
The bond angle in SO2 is 119 degrees. The lone pair takes up more room than a bonding pair, squeezing the bond angle just under the ideal 120 degrees.
What is the hybridization of SO2?
The central S in SO2 is sp2 hybridized. Steric number 3 means 3 orbitals have to be mixed, which is exactly what sp2 gives you.
Is SO2 polar or nonpolar?
SO2 is polar. The S-O bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.
Does SO2 have resonance structures?
Yes. SO2 has two equivalent resonance structures. The real molecule is not any one of them - it is an average, so every bond that differs between the drawings is really the same length in the actual molecule.
Does SO2 obey the octet rule?
It can be drawn either way, and different courses teach different conventions. The octet structure keeps eight electrons on every atom but leaves formal charges behind. The expanded-octet structure puts more than eight electrons on the central atom and brings every formal charge to zero. Both are shown on this page.