Compound reference
Hydrogen cyanide
HCN
The chemistry
A single H–C bond on one end, a triple C≡N bond on the other. Three regions of electron density on C → linear. The triple bond is short, strong, and stiff.
Remember it as…
H–C≡N. The triple bond doesn't bend - and counts as ONE region for VSEPR.
Common mix-up
A triple bond does NOT count as three electron groups. All three pairs sit on the same axis between C and N - one direction of repulsion, one group.
Where the name comes from
Cyan is Greek kyanos (dark blue) - the first cyanide salts gave a deep blue. The blue pigment "Prussian blue" is iron ferrocyanide.
Where you meet it
Famously toxic because the C≡N bond survives long enough in the body to bind the iron in cytochrome-c-oxidase, halting cellular respiration.
PubChem facts
- IUPAC name
- formonitrile
- Molecular weight
- 27.025 g/mol
Also known as: hydrocyanic acid, prussic acid
Handling note
Extremely toxic (it blocks the body's ability to use oxygen) and also flammable. One of the most dangerous on this list.
BondingMolecular geometry
Chemical data from PubChem (NIH/NCBI)