HCN Lewis Structure
Hydrogen cyanide has a linear Lewis structure: hydrogen single-bonded to carbon, carbon triple-bonded to nitrogen (H–C≡N). HCN uses 10 valence electrons, is linear (VSEPR AX₂) with a 180° bond angle, has sp hybridization on both C and N, formal charges of 0 on every atom, 2 σ and 2 π bonds, and is polar overall.
Lewis Structure
Bonds and lone pairs. The linear shape is shown beside it.
Molecular Geometry
Linear shape · 180° · AX₂
| Total valence electrons | 10 |
|---|---|
| Central atom | Carbon (C) |
| C–H | Single bond |
| C≡N | Triple bond |
| Lone pairs on C | 0 |
| Lone pairs on N | 1 |
| Electron geometry | Linear |
| Molecular geometry | Linear |
| VSEPR | AX₂ |
| Bond angle | 180° |
| Polarity | Polar |
| C hybridization | sp |
| N hybridization | sp |
| Sigma bonds | 2 |
| Pi bonds | 2 |
| Formal charges | 0 on every atom |
How to Draw the HCN Lewis Structure
Draw hydrogen cyanide step by step: count 10 valence electrons, put carbon between H and N (not H–N–C), place single bonds, finish nitrogen’s octet, then form the C≡N triple bond so carbon reaches eight electrons too.
Step 1Count the valence electrons
Hydrogen brings 1, carbon 4, nitrogen 5. HCN is neutral, so nothing is added or removed for charge: 1 + 4 + 5 = 10 valence electrons. Every valid Lewis drawing of HCN must place all 10.
Atom Count Valence e⁻ Total H 1 1 1 C 1 4 4 N 1 5 5 Total 10 Step 2Choose the H–C–N skeleton
Hydrogen forms only one bond, so it is never central. Between C and N, carbon is less electronegative and takes the center: H—C—N. H—N—C is the wrong skeleton here — it does not give the H–C≡N structure with formal charges of 0 on every atom.
Step 3Add single bonds
Connect H to C and C to N with single bonds. Those two links use 4 of the 10 valence electrons, leaving 6 electrons still to place.
Step 4Complete the nitrogen octet
Put the remaining 6 electrons on nitrogen as three lone pairs first (outer atoms before the center). Nitrogen then has an octet, but carbon still has only four electrons from the two single bonds — short of eight.
Step 5Form the C≡N triple bond
Move two lone pairs from nitrogen into the C–N link, one pair at a time. Each move raises the bond order: C—N → C=N → C≡N. Stop when carbon has eight electrons. The finished structure is H—C≡N with one lone pair left on nitrogen. Hydrogen has its duet, C and N each have an octet, all 10 electrons are used, and every formal charge is 0.
- Hydrogen duet satisfied
- Carbon octet satisfied
- Nitrogen octet satisfied
- All 10 valence electrons placed
- Formal charges 0 on H, C, and N
HCN Valence Electrons
hydrogen contribute 1, and Carbon contributes 4 valence electrons, and nitrogen contribute 5. Added together, HCN starts with 10 valence electrons to place, and as a neutral molecule there is no charge to add or remove. Of those, 8 fill the bonds and 2 sit as lone pairs (1 on nitrogen).
| hydrogen: 1 valence electron |
| carbon: 4 valence electrons |
| nitrogen: 5 valence electrons |
| Total: 1 + 4 + 5 = 10 |
HCN Lewis Structure with Formal Charges
Formula
Formal charge = valence e⁻ − nonbonding e⁻ − ½ × bonding e⁻
| Atom | Calculation | Formal charge |
|---|---|---|
| H | 1 − 0 − ½(2) = 0 | 0 |
| C | 4 − 0 − ½(8) = 0 | 0 |
| N | 5 − 2 − ½(6) = 0 | 0 |
The formal charges sum to 0, matching the neutral molecule. That makes this the preferred HCN Lewis structure: zero formal charge on every atom.
Formal charge = valence e⁻ − nonbonding e⁻ − ½(bonding e⁻). H: 1 − 0 − ½(2) = 0. C: 4 − 0 − ½(8) = 0. N: 5 − 2 − ½(6) = 0. Every atom is zero and the charges sum to 0, so H–C≡N is the preferred Lewis structure. Charge-separated drawings of HCN are much less important.
HCN Molecular Geometry
| Electron geometry | Linear |
|---|---|
| Molecular geometry / shape | Linear |
| VSEPR notation | AX₂ |
| Bond angle | 180° |
On the central carbon, count electron domains the VSEPR way: each bonded neighbor is one domain (a double or triple bond still counts as one), and each lone pair is one more. HCN has 2 bonding domains and no lone pairs (AX₂), for 2 electron domains in total.
Two domains repel to opposite sides of the carbon, so H─C≡N is linear with a bond angle of 180°. With no lone pairs on the central atom, the electron geometry and the molecular geometry are the same: both linear.
HCN is AX₂: carbon has two bonding domains (to H and to N) and no lone pairs. Electron geometry and molecular geometry are both linear, with a 180° H–C–N angle. CO₂ (O=C=O) and acetylene (H–C≡C–H) follow the same two-domain pattern — sp hybrids on a straight line. Unlike CO₂, HCN’s ends are different atoms, so the linear shape does not make the molecule nonpolar.
Why Is HCN Linear?
A triple bond does not bend HCN. In VSEPR, H–C is one electron domain and C≡N is still one domain, even though three pairs are shared. Carbon has no lone pairs, so those two domains sit 180° apart. The triple bond raises bond order and adds π bonds; it does not add a third domain. Two domains mean a linear molecule, not a bent one.
HCN Bond Angle
The H–C–N bond angle is 180°. With two electron domains and no lone pairs on carbon, the bonds lie on a straight line. sp hybridization gives the same result: two hybrids point 180° apart.
Is HCN Polar or Nonpolar?
HCN is polar.
HCN is linear — and still polar. Polarity depends on the whole molecule, not on the bond angle alone. One end is H–C; the other is C≡N. Nitrogen (electronegativity 3.04) pulls shared electrons harder than carbon (2.55) or hydrogen (2.20), so the bond dipoles along the axis are not equal and opposite. A net molecular dipole remains, pointing toward nitrogen.
CO₂ is also linear AX₂, but both ends are the same C=O unit, so its two bond dipoles cancel and CO₂ is nonpolar. Same linear shape, different outer atoms — that is why HCN is polar while CO₂ is not.
Linear shape, but the two ends are different atoms — bond dipoles do not cancel.
HCN Hybridization
Carbon in HCN is sp-hybridized; nitrogen is sp-hybridized.
| Carbon (C) | sp |
|---|---|
| Nitrogen (N) | sp |
Carbon has two electron domains (C–H and C≡N), so it mixes one 2s orbital with one 2p orbital into two sp hybrids pointing 180° apart. One hybrid forms the σ bond to hydrogen; the other forms the σ bond to nitrogen. The two leftover p orbitals on carbon overlap sideways with p orbitals on nitrogen to make the two π bonds of the triple bond.
Nitrogen is sp-hybridized as well: one sp hybrid holds the lone pair, the other forms the C–N σ bond, and two p orbitals supply the π bonds. Both C and N are sp; that matches the linear 180° geometry.
Sigma and Pi Bonds in HCN
Count σ and π from the finished Lewis structure, not from the molecular formula alone.
H–C single bond → 1 σ (no π).
C≡N triple bond → 1 σ + 2 π.
Whole molecule: 2 σ and 2 π. N₂ is a pure triple bond (1 σ + 2 π); the C≡C unit in acetylene matches that count. HCN keeps the same triple-bond pattern on C≡N and adds one more σ from H–C. The σ framework comes from the sp hybrids; the two π bonds are sideways p–p overlaps on carbon and nitrogen.
| Sigma bonds (σ) | 2 |
|---|---|
| Pi bonds (π) | 2 |
Bonding diagram
- H—C1 σ
- C≡N1 σ + 2 π
- Csp hybridized
The sp hybrid orbitals on carbon make the σ framework; any leftover p orbitals on the multiply bonded atoms form the π bonds.
HCN Lewis Structure Summary
| Chemical name | Hydrogen cyanide |
|---|---|
| Total valence electrons | 10 |
| Central atom | Carbon |
| Structure | H─C≡N |
| C–H | Single bond |
| C≡N | Triple bond |
| Lone pairs on carbon | 0 |
| Lone pairs on outer atoms | 1 on nitrogen |
| Electron geometry | Linear |
| Molecular geometry | Linear |
| VSEPR | AX₂ |
| Bond angle | 180° |
| Hybridization | Carbon sp; Nitrogen sp |
| Polarity | Polar |
| Sigma bonds | 2 |
| Pi bonds | 2 |
| Formal charge | 0 on every atom |
| Resonance | No |
| Overall charge | 0 |
Try Another Lewis Structure
Enter another formula to draw its Lewis structure — bonds, lone pairs, formal charges, and the steps behind them.
Related Lewis Structures
Common next structures from general chemistry — each link opens that molecule so you can compare geometry, polarity, or the drawing steps.
Frequently Asked Questions
What is the Lewis structure of HCN?
The Lewis structure of HCN is H–C≡N: hydrogen single-bonded to carbon, carbon triple-bonded to nitrogen, and one lone pair on nitrogen. It uses all 10 valence electrons, and every formal charge is 0.
How many valence electrons does HCN have?
HCN has 10 valence electrons: 1 from hydrogen, 4 from carbon, and 5 from nitrogen. They fill the H–C single bond, the C≡N triple bond (1 σ + 2 π), and the lone pair on nitrogen.
What is the molecular geometry of HCN?
HCN has a linear molecular geometry. Carbon is central with two bonding domains and no lone pairs (VSEPR AX₂), so H–C≡N is straight with a 180° bond angle.
Why is HCN linear?
Carbon in HCN has two electron domains: the bond to hydrogen and the triple bond to nitrogen. A triple bond still counts as one domain in VSEPR. Two domains arrange 180° apart, so the molecule is linear — the triple bond does not bend it.
Is HCN polar or nonpolar?
HCN is polar. It is linear, but the ends differ (H–C vs C≡N). Nitrogen is more electronegative than carbon and hydrogen, so the bond dipoles along the axis do not cancel and a net molecular dipole remains toward nitrogen.
What is the hybridization of carbon in HCN?
Carbon in HCN is sp-hybridized. Two electron domains mix one s and one p orbital into two sp hybrids 180° apart; the remaining p orbitals form the π bonds of C≡N. Nitrogen in HCN is also described as sp-hybridized.
How many sigma and pi bonds are in HCN?
HCN has 2 sigma bonds and 2 pi bonds. H–C is 1 σ. C≡N is 1 σ + 2 π.
Does HCN have resonance structures?
HCN is drawn as a single structure, H–C≡N, with formal charges of 0 on every atom. Charge-separated alternatives matter far less than that dominant form, so HCN is not a classic resonance example the way O₃ or NO₃⁻ is.
What is the bond angle of HCN?
The H–C–N bond angle is 180°. With two electron domains and no lone pairs on carbon, the bonds sit directly opposite each other.