SO32− Lewis Structure
Sulfite ion (sulphite) · The preservative in wine and dried fruit, and the anion of sulfurous acid.
The SO3 2- Lewis structure has 26 valence electrons - the 2− charge adds two, drawn as three S–O bonds, with ten lone pairs in total, all inside square brackets. The central S is sp3 hybridized, which makes it a trigonal pyramidal ion with a bond angle of 107 degrees. Each of the three O atoms carries a 1− formal charge and the S a 1+; add them up and you get 2−, the charge on the ion. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean.
- 26 valence e−
- trigonal pyramidal
- ≈107° bond angle
- sp3
- 2− charge
SO32− molecular geometry
Trigonal pyramidal, from 4 electron domains on the central S: 3 bonded atoms and 1 lone pair (AX3E1). The electron geometry is tetrahedral; the shape is what is left when the lone pairs are not counted as atoms. Wedges point toward you, hashed wedges away. Every shape in the VSEPR chart →
Calculated properties
| Total valence electrons | 26 |
|---|---|
| Bonding electrons | 6 (3 shared pairs) |
| Nonbonding electrons | 20 (10 lone pairs) |
| Lone pairs on S | 1 |
| Electron domains (steric number) | 4 |
| VSEPR notation | AX3E1 |
| Electron geometry | tetrahedral |
| Molecular geometry | trigonal pyramidal |
| Bond angle | ≈107° |
| Hybridization | sp3 |
| Charge | 2− |
| Charge distribution | localised |
| Formal charges | S: +1, O: -1, O: -1, O: -1 |
| Resonance structures | none |
| Molar mass | 80.057 g/mol |
S Sulfur 6 valence e− O Oxygen 3 atoms, 6 valence e− each
How to draw the SO32− Lewis structure
Step through it. The bar shows how much of the 26-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. 3 bonds spends 6 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 8 electrons around it.
The reasoning at each step
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Count the valence electrons
Add up the outer-shell electrons every atom brings, then account for the charge. A 2- ion has gained two electrons, so they are added to the budget. Skip this and every count that follows will be wrong.
6 (S) + 3 x 6 (O) + 2 (2- charge) = 26 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.
3 bonds x 2 = 6 electrons used, 20 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 20, has 20, exactly enough -
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: 3, O3: 3 (20 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, and they are not the ion's charge either - but they must add up to it. Adding these up gives 2-, which is the check that the structure is complete.
S: 6 - 2 - 3 = +1 ; O1: 6 - 6 - 1 = -1 ; O2: 6 - 6 - 1 = -1 ; O3: 6 - 6 - 1 = -1 -
Work out the shape
Count the electron domains on S: 3 bonded groups plus 1 lone pair. A double or triple bond still counts as one domain, because it points in one direction. That gives tetrahedral electron geometry; ignore the lone pairs and the atoms themselves sit in a trigonal pyramidal arrangement. One lone pair pushes the three bonds down from the ideal 109.5 degrees.
steric number 4 -> sp3 -> trigonal pyramidal, bond angle 107 -
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
The expanded-octet version
Courses disagree about SO3 2-. 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.
Where the 2− charge sits
Each of the three O atoms carries a 1− formal charge and the S a 1+; add them up and you get 2−, the charge on the ion. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean.
People also ask whether SO32− is polar. Strictly it is neither: polar and nonpolar sort neutral molecules by whether their bond dipoles cancel. A 2− ion is pulled towards any nearby charge whatever its shape, which is why its salts dissolve in water regardless of geometry. The table below is still worth reading - it shows how unevenly each bond holds its electrons, which is what decides whether the charge ends up spread or concentrated.
| Bond | Electronegativity difference | Character |
|---|---|---|
| S–O | 0.86 | polar covalent |
Common questions
How many valence electrons does SO3 2- have?
SO3 2- has 26 valence electrons. The atoms bring 24 between them, and the 2− charge adds two. 6 of them are in bonds and 20 sit in lone pairs.
Why does SO3 2- have a 2− charge?
Each of the three O atoms carries a 1− formal charge and the S a 1+; add them up and you get 2−, the charge on the ion. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean. The brackets around the drawing are part of the answer: they say the charge belongs to the whole ion rather than to whichever atom happens to be nearest.
What is the molecular geometry of SO3 2-?
SO3 2- is a trigonal pyramidal ion. The central S has 3 bonded groups and one lone pair, a steric number of 4, which gives tetrahedral electron geometry and a trigonal pyramidal ion.
What is the bond angle in SO3 2-?
The bond angle in SO3 2- is 107 degrees. One lone pair pushes the three bonds down from the ideal 109.5 degrees.
What is the hybridization of SO3 2-?
The central S in SO3 2- is sp3 hybridized. Steric number 4 means 4 orbitals have to be mixed, which is exactly what sp3 gives you.
Is SO3 2- polar or nonpolar?
Neither, strictly. Polar and nonpolar describe neutral molecules, by asking whether their bond dipoles cancel out. SO3 2- carries a net 2− charge, so it is pulled towards any nearby charge whatever its shape - which is why ionic compounds dissolve in water regardless of geometry. The question the shape does answer is whether the charge is spread evenly: it is not, because 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 SO3 2- 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.