Numford
Lewis structure

Lewis structure of CO₃²⁻

24 valence electrons. One C=O double bond and two C-O single bonds, with no lone pair on C. 3 resonance structures.

OCOO
  • C
  • O

CO₃²⁻Carbonate ion

Trigonal planar

AB₃: 3 bond pairs, no lone pairs

  • Hybridisationsp²
  • Bond angle120°
  • Polarityion, charge −2

Lewis structure

24 valence electrons, 3 resonance structures
i

NCERT draws these three resonance structures; the real ion has three equal C-O bonds.

COOO−−
COOO−−
COOO−−

Formal charges: O −1 on two atoms

Why this shape

  1. C has 4 valence electrons. It uses four in one double bond to O and two single bonds to O, so none are left over.
  2. Steric number = three sigma bonds + no lone pairs = 3, so C is sp² hybridised. A double or triple bond counts once: only its sigma bond uses a hybrid orbital.
  3. 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.
  4. All the bond pairs repel each other equally, so the angle is the ideal 120°.
  5. CO₃²⁻ is an ion with an overall charge of −2, so it is not described as polar or non-polar.
Working

How to draw it

  1. Count the valence electrons: C 4 + 3 × O 6 + 2 for the ²⁻ charge = 24 valence electrons, 12 pairs.
  2. Put C in the middle and join it to the three outer atoms with single bonds.
  3. 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.
  4. 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.
  5. Result: one C=O double bond and two C-O single bonds, with no lone pair on C.
  6. Formal charge = valence electrons − lone pair electrons − number of bonds. Here: O −1 on two atoms.
  7. The double bond can sit in three equivalent ways, so there are three resonance structures. The real ion is a hybrid of them.

Class 11, Chapter 4 Chemical Bonding and Molecular Structure.

Questions

Lewis structure of CO3 2-: quick answers

How many valence electrons are in CO3 2-?

C 4 + 3 × O 6 + 2 for the ²⁻ charge = 24 valence electrons, 12 pairs.

What are the formal charges in the Lewis structure of CO3 2-?

O −1 on two atoms; every other atom is zero. Formal charge = valence electrons - lone pair electrons - number of bonds.

How many resonance structures does CO3 2- have?

3. The double bond can sit in three equivalent ways, so there are three resonance structures. The real ion is a hybrid of them.

Try any molecule