The Lewis Dot Structure for HCN

HCN Lewis Structure
Created by MakeTheBrainHappy

The molecule HCN has one of the most fascinating lewis structures. It is one of the few which features a nitrogen triple bond. It is possible to draw the structure with two electrons in place of the lines to represent the covalent bonds, which would result in there being six shared electrons between the carbon and nitrogen. We know that hydrogen can only share one bond with the carbon as we looked at in a previous article. Hydrogen essentially only needs two valence electrons to fill its outer shell because it only has a 1s orbital. The carbon, based on its position in the periodic table, has four electrons to share which it does with the nitrogen. The total number of "shared" electrons equals eight, filling carbon's valence shell. The nitrogen's "shared electron" count is six, which means that it needs two more electrons to fill its shell which is accounted for by the lone pair. Furthermore, this brings the total number of electrons on nitrogen to five as the periodic table predicts.

HCN has a linear geometry but unlike nonpolar CO2 is polar due to the intrinsic electronegativity on nitrogen and the stronger force by Coulomb's law pulling on the electrons in the triple bond creating a more negative bent on the right (based on the diagram) than the left. This means that it undergoes dipole-dipole interactions not unlike water (h2o). Water's intermolecular bonds are considered to be stronger hydrogen bonds (H-F, H-N, H-O bonds) due to these having the largest electronegativity difference. By this logic, HCN dipole-dipole interactions are also weaker the intramolecular bonds in network covalent solids. Nevertheless, these stronger intermolecular forces contribute to its boiling point at room temperature, or approximately 26˚C.
Structure of HCN from Wikimedia Commons. Source

While HCN is produced as a precursor to many molecules and chemicals, the compound itself is extremely flammable and poisonous. It can be produced by combining hydrogen chloride (HCl) and Sodium Cyanide (NaCN). Often described as a pale or blue colorless liquid, it has a distinctive scent that will warn those of its toxicity. Furthermore, it is a weak acid that partially ionizes in water. 

HCN's structure also features an sp hybridized structure with 180˚ of difference between the two molecules on the end. The triple bond contains one sigma bond and two pi bonds. Such a simple molecule can in the end still contain more complex chemistry.



  1. Could it be written H-C-(3 lines)N:? I don't understand the difference between using the dots and lines.

    1. Each dot represents an electron, whereas each line represents two electrons, shared between the two atoms that have the line between them.

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