HCN has a hydrogen atom single-bonded to a carbon atom, and that carbon atom is triple-bonded to a nitrogen atom.
These are all non-metals, so the bonds are covalent and HCN is therefore a covalent (aka Molecular) structure.
Carbon brings four valence electrons with it; it needs four more to complete its valence shell. Hydrogen shares one electron with it, and nitrogen shares three. This completes carbon's octet.
Carbon likewise shares one electron back with Hydrogen (this complete's hydrogen's outer shell of two electrons, aka Doublet) and carbon shares three electrons back with Nitrogen. This completes nitrogen's octet.
You can watch this structure get drawn below, or you can scroll to the bottom of this page for a completed structure.
What is the hybridization of Carbon in HCN?
Carbon is triple-bonded to nitrogen, and so there are two pi bonds (Remember: The first bond between any two atoms is a sigma bond, and the second/third bonds are pi bonds). This means two p orbitals are required to be left over after hybridization.
2 pi bonds = 2 leftover p orbitals.
This means only ONE of carbon's p orbitals is available to hybridize, and so the hybridization of C in HCN is "sp".
What is the hybridization of N in HCN?
Nitrogen is triple-bonded to carbon, and so two pi bonds are required here as well. This means only one of nitrogen's p orbitals is available to be hybridized, and so the hybridization of nitrogen in HCN is "sp".
What is the molecular shape (VSEPR shape) of HCN?
Because the carbon is connected to two atoms, with no lone pairs on that central carbon, the geometry is AX2, which is "linear". The bond angle is 180 degrees.