For the polyatomic ion, carbonate, CO32:
(A) Draw the Lewis structure of carbonate.
(B) How many total valence electrons are there in the polyatomic ion?
(C) What is the steric number?
(D) Does carbonate have any resonance structures (yes or no)?
(E) If carbonate has any resonance structures, draw them on your work sheet.
(F) On your work, draw carbonate using perspective drawing (include any lone-pair electrons on the central atom that affect the shape of the molecule; do not include electrons on the surrounding atoms or draw multiple bonds if present).
(G) What is the electron geometry?
(H) What is the molecule shape?
(1) Is carbonate polar or nonpolar?
The correct answer and explanation is :
Answering the Questions on Carbonate Ion, ( CO_3^{2-} ):
(A) Lewis Structure of Carbonate Ion (( CO_3^{2-} ))
The Lewis structure for carbonate consists of a central carbon atom single-bonded to two oxygen atoms and double-bonded to one oxygen atom. Each singly bonded oxygen carries a negative charge, giving the ion an overall charge of -2.
(B) Total Valence Electrons
- Carbon (C) has 4 valence electrons.
- Oxygen (O) has 6 valence electrons (3 oxygen atoms contribute ( 3 \times 6 = 18 ) electrons).
- The ion has a -2 charge, meaning we add 2 extra electrons.
- Total valence electrons = ( 4 + 18 + 2 = 24 ).
(C) Steric Number
The steric number is the number of bonded atoms plus the number of lone pairs on the central atom. Carbon in ( CO_3^{2-} ) forms three sigma bonds with oxygen and has no lone pairs, so its steric number is 3.
(D) Does Carbonate Have Resonance?
Yes, the carbonate ion exhibits resonance, meaning the double bond can shift among the three oxygen atoms.
(E) Resonance Structures
Carbonate has three resonance structures where the double bond shifts between the oxygen atoms.
(F) Perspective Drawing
The carbonate ion is planar, and all oxygens are equivalent due to resonance.
(G) Electron Geometry
The electron domain geometry is trigonal planar.
(H) Molecular Shape
Since there are three bonding groups and no lone pairs on the central carbon, the molecular shape is trigonal planar.
(I) Is Carbonate Polar or Nonpolar?
The carbonate ion is nonpolar. Even though individual C=O bonds are polar, the molecule is symmetrical in a trigonal planar shape. This symmetry causes dipole moments to cancel out, resulting in a nonpolar overall charge distribution.
