C2O42− Lewis Structure
Oxalate ion · Found in rhubarb and spinach, and the main component of most kidney stones.
The C2O4 2- Lewis structure has 34 valence electrons - the 2− charge adds two, drawn as one C–C bond, two C–O bonds and two C=O bonds, with ten lone pairs in total, all inside square brackets. The central C is sp2 hybridized, which makes it a trigonal planar ion with a bond angle of 120 degrees. The 2− charge does not sit on one atom. Across the four resonance structures each of the four O atoms carries it in turn, so the real ion spreads it evenly over all four - and every bond between them is identical, whatever a single drawing suggests. There are four equivalent resonance structures.
- 34 valence e−
- trigonal planar
- 120° bond angle
- sp2
- 2− charge
C2O42− molecular geometry
Trigonal planar, from 3 electron domains on the central C: 3 bonded atoms (AX3). The electron geometry is trigonal planar. Wedges point toward you, hashed wedges away. Every shape in the VSEPR chart →
Calculated properties
| Total valence electrons | 34 |
|---|---|
| Bonding electrons | 14 (7 shared pairs) |
| Nonbonding electrons | 20 (10 lone pairs) |
| Lone pairs on C | 0 |
| Electron domains (steric number) | 3 |
| VSEPR notation | AX3 |
| Electron geometry | trigonal planar |
| Molecular geometry | trigonal planar |
| Bond angle | 120° |
| Hybridization | sp2 |
| Charge | 2− |
| Charge distribution | delocalised over the resonance forms |
| Formal charges | O: -1, O: -1 |
| Resonance structures | 4 |
| Molar mass | 88.018 g/mol |
C Carbon 2 atoms, 4 valence e− each O Oxygen 4 atoms, 6 valence e− each
How to draw the C2O42− Lewis structure
Step through it. The bar shows how much of the 34-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. 5 bonds spends 10 electrons.
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Outer octets. Spend electrons as lone pairs on the outer atoms. That uses the entire budget, and the central atom has nothing left over.
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Multiple bonds. The central atom is still short of an octet, so two lone pairs on the outer atoms move 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, 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.
2 x 4 (C) + 4 x 6 (O) + 2 (2- charge) = 34 valence electrons -
Work out what is bonded to what
Oxalate is ⁻OOC–COO⁻: two carboxylate groups joined by a carbon–carbon bond, with two oxygens on each carbon.
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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.
5 bonds x 2 = 10 electrons used, 24 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 28, has 24, short by 4 -
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 4 is covered by 2 extra shared pairs. That is where the double bonds come from.
C1=O2, C2=O4 -
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.
O1: 3, O2: 2, O3: 3, O4: 2 (20 electrons) -
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.
O1: 6 - 6 - 1 = -1 ; O3: 6 - 6 - 1 = -1 -
Work out the shape
Count the electron domains on C: 3 bonded groups. 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 trigonal planar arrangement.
steric number 3 -> sp2 -> trigonal planar, bond angle 120 -
Decide whether it is polar
The bond dipoles and lone pairs are arranged symmetrically, so the vectors cancel and the molecule has no net dipole.
dipoles cancel -> nonpolar
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 ion. C2O4 2- is an average of the 4 structures below, which is why bonds that look different here are actually identical in the real ion.
Where the 2− charge sits
The 2− charge does not sit on one atom. Across the four resonance structures each of the four O atoms carries it in turn, so the real ion spreads it evenly over all four - and every bond between them is identical, whatever a single drawing suggests.
People also ask whether C2O42− 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 |
|---|---|---|
| C–O | 0.89 | polar covalent |
| C–C | 0.00 | nonpolar covalent |
Common questions
How many valence electrons does C2O4 2- have?
C2O4 2- has 34 valence electrons. The atoms bring 32 between them, and the 2− charge adds two. 14 of them are in bonds and 20 sit in lone pairs.
Why does C2O4 2- have a 2− charge?
The 2− charge does not sit on one atom. Across the four resonance structures each of the four O atoms carries it in turn, so the real ion spreads it evenly over all four - and every bond between them is identical, whatever a single drawing suggests. 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 C2O4 2-?
C2O4 2- is a trigonal planar ion. The central C has 3 bonded groups, a steric number of 3, which gives trigonal planar electron geometry and a trigonal planar ion.
What is the bond angle in C2O4 2-?
The bond angle in C2O4 2- is 120 degrees. That is the ideal trigonal planar angle, and nothing distorts it here.
What is the hybridization of C2O4 2-?
The central C in C2O4 2- is sp2 hybridized. Steric number 3 means 3 orbitals have to be mixed, which is exactly what sp2 gives you.
Is C2O4 2- polar or nonpolar?
Neither, strictly. Polar and nonpolar describe neutral molecules, by asking whether their bond dipoles cancel out. C2O4 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, because the trigonal planar arrangement puts every identical bond at the same angle to the others.
Does C2O4 2- have resonance structures?
Yes. C2O4 2- has four equivalent resonance structures. The real ion 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 ion.