C2H3O2− Lewis Structure
Acetate ion (ethanoate, CH3COO-) · What vinegar becomes when it loses its proton, and a common buffer in biochemistry.
The C2H3O2- Lewis structure has 24 valence electrons - the 1− charge adds one, drawn as one C–C bond, one C–O bond, one C=O bond and three bonds to hydrogen, with five 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 1− charge does not sit on one atom. Across the two resonance structures each of the two O atoms carries it in turn, so the real ion spreads it evenly over both - and every bond between them is identical, whatever a single drawing suggests. There are two equivalent resonance structures.
- 24 valence e−
- trigonal planar
- 120° bond angle
- sp2
- 1− charge
C2H3O2− 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 | 24 |
|---|---|
| Bonding electrons | 14 (7 shared pairs) |
| Nonbonding electrons | 10 (5 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 | 1− |
| Charge distribution | delocalised over the resonance forms |
| Formal charges | O: -1 |
| Resonance structures | 2 |
| Molar mass | 59.044 g/mol |
C Carbon 2 atoms, 4 valence e− each H Hydrogen 3 atoms, 1 valence e− each O Oxygen 2 atoms, 6 valence e− each
How to draw the C2H3O2− Lewis structure
Step through it. The bar shows how much of the 24-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. 6 bonds spends 12 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 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, then account for the charge. A 1- ion has gained one electron, so it is added to the budget. Skip this and every count that follows will be wrong.
2 x 4 (C) + 3 x 1 (H) + 2 x 6 (O) + 1 (1- charge) = 24 valence electrons -
Work out what is bonded to what
Acetate is CH3–COO⁻: three hydrogens on one carbon, both oxygens on the other. The charge is shared between the two oxygens, not held by either one.
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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.
6 bonds x 2 = 12 electrons used, 12 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 14, has 12, 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.
C2=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.
O1: 3, O2: 2 (10 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 1-, which is the check that the structure is complete.
O1: 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 atoms around the centre are not all the same, so the bond dipoles are different sizes and cannot 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 ion. C2H3O2- is an average of the 2 structures below, which is why bonds that look different here are actually identical in the real ion.
Where the 1− charge sits
The 1− charge does not sit on one atom. Across the two resonance structures each of the two O atoms carries it in turn, so the real ion spreads it evenly over both - and every bond between them is identical, whatever a single drawing suggests.
People also ask whether C2H3O2− is polar. Strictly it is neither: polar and nonpolar sort neutral molecules by whether their bond dipoles cancel. A 1− 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–H | 0.35 | nonpolar covalent |
| C–C | 0.00 | nonpolar covalent |
Common questions
How many valence electrons does C2H3O2- have?
C2H3O2- has 24 valence electrons. The atoms bring 23 between them, and the 1− charge adds one. 14 of them are in bonds and 10 sit in lone pairs.
Why does C2H3O2- have a 1− charge?
The 1− charge does not sit on one atom. Across the two resonance structures each of the two O atoms carries it in turn, so the real ion spreads it evenly over both - 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 C2H3O2-?
C2H3O2- 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 C2H3O2-?
The bond angle in C2H3O2- is 120 degrees. That is the ideal trigonal planar angle, and nothing distorts it here.
What is the hybridization of C2H3O2-?
The central C in C2H3O2- is sp2 hybridized. Steric number 3 means 3 orbitals have to be mixed, which is exactly what sp2 gives you.
Is C2H3O2- polar or nonpolar?
Neither, strictly. Polar and nonpolar describe neutral molecules, by asking whether their bond dipoles cancel out. C2H3O2- carries a net 1− 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 atoms around the centre are not all the same, so the bond dipoles are different sizes and cannot cancel.
Does C2H3O2- have resonance structures?
Yes. C2H3O2- has two 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.