Hydrogen cyanide Lewis Structure

Hydrogen Cyanide Lewis Structure: HCN Lewis Structure, Linear Geometry, sp Hybridization & Polarity

Hydrogen cyanide Key Facts

The hydrogen cyanide Lewis structure is H–C≡N, a single H–C bond, a C≡N triple bond, and one lone pair on nitrogen; the molecule is linear, carbon is sp-hybridized, the bond axis is 180°, and because the very polar C≡N bond is not balanced, HCN is polar.

Figure 1: Lewis Structure of Hydrogen cyanide (HCN)

Chemical Formula	HCN
CAS Number	74-90-8
Total Valence Electrons	10
Lewis Structure	H–C≡N, one lone pair on N
Electron-Domain Geometry	Linear
Molecular Geometry	Linear
Hybridization Model	sp (C and N)
H–C≡N Bond Angle	180°
Polarity	Polar molecule

PubChem and NIST identify hydrogen cyanide as HCN with CAS 74-90-8; PubChem lists a molecular weight of approximately 27.025 g/mol. View CAS 74-90-8 dictionary details on Guidechem →

Applications of Hydrogen cyanide (CAS 74-90-8)

Hydrogen cyanide is a compact but strategically important building block in fine and commodity chemicals. Its largest use is in the production of adiponitrile, the key intermediate for nylon 66 (via hexamethylenediamine), traditionally made by hydrocyanation of butadiene.

HCN is also a precursor to acrylate and methacrylate esters (via acetone cyanohydrin routes to methyl methacrylate), to sodium cyanide and potassium cyanide for mining cyanidation and metal plating, and to chelating agents such as EDTA. It is used in fumigation and certain specialty syntheses, always under tightly controlled, contained conditions.

Step 1 - Valence Electrons & Central Atom

Count valence electrons for HCN: hydrogen contributes 1 valence electron, carbon (Group 14) contributes 4, and nitrogen (Group 15) contributes 5.

Step 2 - Lewis Structure & Electron Distribution

Start with H–C–N. A single bond to H (2 e) and a single C–N bond (2 e) consume 4 electrons, leaving 6. Placing those as lone pairs leaves nitrogen with only a single bond (incomplete octet) and carbon with too few electrons.

Forming a C≡N triple bond resolves this: H–C≡N. The triple bond uses 6 electrons, the remaining 2 electrons become one lone pair on nitrogen. Now carbon shares 8 electrons (one single + one triple), nitrogen has a triple bond plus a lone pair (8 electrons), and hydrogen has its duet. Formal charges are zero.

Step 3 - VSEPR Theory & Geometry

The Lewis structure gives two electron domains around the central carbon: the H–C bonding domain and the C≡N bonding domain (a triple bond counts as one domain). Two domains adopt a linear arrangement, so the H–C≡N axis is linear at 180°.

There are no lone pairs on carbon, so the electron-domain and molecular geometries are both linear. Nitrogen's lone pair lies along the molecular axis and does not bend the molecule.

Hybridization & Formal Charge

In the localized-bond model, both carbon and nitrogen in HCN are sp-hybridized. Carbon uses one sp hybrid for the H–C σ bond and the other for one component of the C≡N bond; the two unhybridized p orbitals form the two π components of the triple bond. Nitrogen similarly uses sp hybrids for its part of the C≡N bond and its lone pair, with p orbitals for the π bonds.

This sp arrangement explains the linear geometry and the short, strong C≡N triple bond.

Is Hydrogen cyanide Polar or Nonpolar?

Hydrogen cyanide is polar. The C≡N triple bond is strongly polar because nitrogen is much more electronegative than carbon, creating a large bond dipole pointing toward nitrogen. The H–C bond is weakly polar in the opposite sense, but it cannot balance the dominant C≡N dipole.

Because HCN is linear, the net dipole lies along the molecular axis pointing toward nitrogen, giving the molecule a substantial permanent dipole moment (about 2.98 D).

This strong polarity, combined with the acidic character of HCN and the electrophilic carbon of the nitrile, underpins its reactivity in cyanohydrin formation and hydrocyanation. It is also why HCN is both a powerful building block and a severe acute toxin.

Structural Comparison Table

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