Skip to content
ScienceQuest

Hydrogen cyanide (HCN) Lewis structure

10 valence electrons, linear

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

Build HCN step by step in the Lewis Structure Generator

What is the Lewis structure of HCN?

Hydrogen cyanide, HCN, has 10 valence electrons. Carbon is the central atom, joined by one C≡N triple bond and one C−H single bond. It carries one lone pair on the N. Every formal charge is zero. C is AX₂: linear, with an ideal bond angle of 180°, and the molecule is polar.

Valence electrons in HCN

Element Group Valence electrons Atoms Electrons
Hydrogen 1 1 1 1
Carbon 14 4 1 4
Nitrogen 15 5 1 5
Total 10

In one line: 1 + 4 + 5 = 10. Each atom brings the valence electrons its group gives it.

How to draw the HCN Lewis structure

  1. Count the valence electrons. Valence electrons: H 1, C 4 and N 5. 1 + 4 + 5 = 10.
  2. Choose the central atom. C goes in the middle: C is the least electronegative atom other than H, χ 2.55 against 3.04 for N. Checked the other way, with N in the middle the formal charges add up to 2, against 0 with C.
  3. Join the atoms with single bonds. Two single bonds use 4 electrons, leaving 10 − 4 = 6.
  4. Complete the outer atoms. The outer N takes 6 electrons as lone pairs to reach eight, leaving 6 − 6 = 0.
  5. Put the rest on the central atom. Nothing is left, so C gets no lone pair, and C now has 4 electrons round it.
  6. Make multiple bonds where needed. C has only 4 electrons, so two lone pairs move into the bonds, making a C≡N bond, and C now has 8.

Lone pairs and formal charges in HCN

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
C (central) 1 one triple bond and one single bond 0 8 4 − 0 − 4 = 0
H 1 one single bond 0 2 1 − 0 − 1 = 0
N 1 one triple bond 1 8 5 − 2 − 3 = 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 HCN

Central atom C
VSEPR class AX₂
Electron geometry linear
Molecular geometry linear
Ideal bond angle 180°
Polarity, an estimate polar

Bonds of different polarity do not cancel in this shape.

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 HCN