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ScienceQuest

Acetylene (C₂H₂) Lewis structure

10 valence electrons, linear

C C H H
C₂H₂ as the Lewis Structure Generator draws it: lone pairs as dots.

Build C₂H₂ step by step in the Lewis Structure Generator

What is the Lewis structure of C₂H₂?

Acetylene, C₂H₂, has 10 valence electrons. The two C atoms bond to each other, and the molecule has one C≡C triple bond and two C−H single bonds. It carries no lone pairs. Every formal charge is zero. Each C is AX₂: linear, with an ideal bond angle of 180°, and the molecule is nonpolar.

Valence electrons in C₂H₂

Element Group Valence electrons Atoms Electrons
Carbon 14 4 2 8
Hydrogen 1 1 2 2
Total 10

In one line: 2 × 4 + 2 × 1 = 10. Each atom brings the valence electrons its group gives it.

How to draw the C₂H₂ Lewis structure

  1. Count the valence electrons. Valence electrons: each C 4 and each H 1. 2 × 4 + 2 × 1 = 10.
  2. Bond the two central atoms. C appears twice, so the two C atoms bond to each other and share the H atoms evenly between them.
  3. Join the atoms with single bonds. Three single bonds use 6 electrons, leaving 10 − 6 = 4.
  4. Complete the outer atoms. Every outer atom is H, and its one bond already gives it the two electrons it holds. 4 remain.
  5. Put the rest on the central atom. The 4 left go on the central atoms as two lone pairs on one C, and one C now has 8 electrons round it and the other 4.
  6. Make multiple bonds where needed. One C has only 4 electrons, so two lone pairs move into the bonds, making a C≡C bond, and both C now have 8.

Lone pairs and formal charges in C₂H₂

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
C (central) 2 one triple bond and one single bond 0 8 4 − 0 − 4 = 0
H 2 one single bond 0 2 1 − 0 − 1 = 0

V is the atom’s valence electrons, N its nonbonding electrons and B its bonding electrons. The formal charges add up to 0, the molecule’s overall charge.

Shape and polarity of C₂H₂

Central atoms each C
VSEPR class AX₂
Electron geometry linear
Molecular geometry linear
Ideal bond angle 180°
Polarity, an estimate nonpolar

The two halves are identical and point opposite ways, so their bond dipoles cancel.

Each C has two σ bonds and no lone pairs, two electron domains in all, so its electron geometry is linear. With no lone pairs, the molecular geometry is the same.

The VSEPR Molecular Geometry viewer turns the AX₂ shape in 3D, lone pairs included.

Work with C₂H₂