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ScienceQuest

Acetylide ion (C₂²⁻) Lewis structure

10 valence electrons, linear, a charge of −2

C C − − 2−
C₂²⁻ as the Lewis Structure Generator draws it: lone pairs as dots, nonzero formal charges circled.

Build C₂²⁻ step by step in the Lewis Structure Generator

What is the Lewis structure of C₂²⁻?

Acetylide ion, C₂²⁻, has 2 × 4 + 2 = 10 valence electrons. Its two atoms are joined by one C≡C triple bond. It carries one lone pair on each C. Its formal charges are −1 on each C. As two atoms it is linear.

Valence electrons in C₂²⁻

Element Group Valence electrons Atoms Electrons
Carbon 14 4 2 8
Charge of 2− +2
Total 10

In one line: 2 × 4 + 2 = 10. Each atom brings the valence electrons its group gives it, and the negative charge adds one electron for each unit.

How to draw the C₂²⁻ Lewis structure

  1. Count the valence electrons. Valence electrons: each C 4, and the 2− charge adds 2. 2 × 4 + 2 = 10.
  2. Join the two atoms. With only two atoms there is no choice to make: they bond to each other.
  3. Join the atoms with single bonds. One single bond uses 2 electrons, leaving 10 − 2 = 8.
  4. Place the lone pairs. Neither atom is an outer one, so the 8 left go on the two atoms as three lone pairs on one C and one on the other. Now one C has 8 electrons round it and the other 4.
  5. Make multiple bonds where needed. One C has only 4 electrons, so two lone pairs move into the bond, making C≡C, and both C now have 8.

Lone pairs and formal charges in C₂²⁻

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
C 2 one triple bond 1 8 4 − 2 − 3 = −1

V is the atom’s valence electrons, N its nonbonding electrons and B its bonding electrons. The formal charges add up to −2, the charge on the ion.

Shape and polarity of C₂²⁻

Central atom none, two atoms
Electron geometry linear
Molecular geometry linear
Ideal bond angle none, two atoms
Polarity, an estimate not applicable

An ion carries a net charge, so whether its bond dipoles cancel is not the question polarity asks.

C₂²⁻ Lewis structure compared with its molecular orbital diagram

Lewis structure Molecular orbital diagram
Bond order 3 3
Unpaired electrons 0 0

In the molecular orbital diagram C₂²⁻’s 10 valence electrons put 8 in bonding orbitals and 2 in antibonding ones, so its bond order is (8 − 2)/2 = 3. The Lewis structure gives the same bond order and the same number of unpaired electrons. The Molecular Orbital Diagram Visualizer draws the diagram level by level.

Work with C₂²⁻