Question

Is HCN linear, tetrahedral, or trigonal planar?

Answer 1

This species is linear.

Explanation:

Hydrogen cyanide is a linear molecule. A Lewis formulation counts 1 electron from the hydrogen, 4 electron from the carbon, and 5 electron from the nitrogen, so 5 electron pairs to distribute.

We get `H-C-=N:`; there are 2 regions of electron density around the central carbon (we ignore the double bonds), for which VSEPR predicts linear geometry, and all the bound atoms are neutral. Can you tell me the geometry of acetylene, `H-C-=C-H`?