HCN Lewis Structure

Hydrogen cyanide (prussic acid) · A highly toxic gas used industrially to make plastics and pesticides.

• 10 valence e−
• linear
• 180° bond angle
• sp
• polar

Calculated properties

How to draw the HCN Lewis structure

1. Count the valence electrons

   Add up the outer-shell electrons every atom brings. That total is the budget for the whole structure - every line and every dot has to come out of it, and nothing may be added.

   1 (H) + 4 (C) + 5 (N) = 10 valence electrons

2. Choose the central atom

   Carbon is the central atom - it forms four bonds and is never terminal in these molecules.

3. Join everything with single bonds first

   Every connection starts as one shared pair. Draw them all before worrying about double bonds - the arithmetic in the next step is what tells you where the double bonds have to go.

   2 bonds x 2 = 4 electrons used, 6 left

4. Work out how far short you are

   Add up what every atom still needs to fill its shell - eight electrons for most atoms, two for hydrogen - minus what its single bonds already give it. Compare that with the electrons you have left. The difference decides everything that follows.

   needs 10, has 6, short by 4

5. Turn the shortfall into multiple bonds

   Being short means atoms have to share more. Every pair that moves from a lone pair into a bond counts twice - once for each atom - so a shortfall of 4 is covered by 2 extra shared pairs. That is where the triple bond comes from.

   C#N

6. Fill in the lone pairs

   Every electron not in a bond sits as a lone pair on an atom. Each atom takes exactly what it needs to finish its shell - there is no choice left at this point.

   N: 1 (2 electrons)

7. Check the central atom

   C ends up with a full octet. Every electron in the budget is now placed.

   C: 8 bonding + 0 nonbonding = 8

8. Check the formal charges

   Formal charge is valence electrons, minus lone-pair electrons, minus the number of bonds. Every atom here comes out at zero, which is the sign of a good structure.

   all formal charges = 0

9. Work out the shape

   Count the electron domains on C: 2 bonded groups. A double or triple bond still counts as one domain, because it points in one direction. That gives linear electron geometry; ignore the lone pairs and the atoms themselves sit in a linear arrangement.

   steric number 2 -> sp -> linear, bond angle 180

10. Decide whether it is polar

    The atoms around the centre are not all the same, so the bond dipoles are different sizes and cannot cancel.

    dipoles do not cancel -> polar

Is HCN polar or nonpolar?

HCN is polar. The atoms around the centre are not all the same, so the bond dipoles are different sizes and cannot cancel.

Common questions

How many valence electrons does HCN have?

HCN has 10 valence electrons. 8 of them are in bonds and 2 sit in lone pairs.

What is the molecular geometry of HCN?

HCN is a linear molecule. The central C has 2 bonded groups, a steric number of 2, which gives linear electron geometry and a linear molecule.

What is the bond angle in HCN?

The bond angle in HCN is 180 degrees. That is the ideal linear angle, and nothing distorts it here.

What is the hybridization of HCN?

The central C in HCN is sp hybridized. Steric number 2 means 2 orbitals have to be mixed, which is exactly what sp gives you.

Is HCN polar or nonpolar?

HCN is polar. The atoms around the centre are not all the same, so the bond dipoles are different sizes and cannot cancel.

Related molecules

• CO₂ also built around C
• C₂H₂ also built around C
• CO also built around C
• N₂ also linear
• N₂O also linear
• O₂ also linear