So, the compound is HCN.
H can only form one bond, while carbon can form 4 bonds and N can form three
bons.
That drives you to conclude that H will bond the carbon and the carbon will form a triple bond
with N:
H - C (triple bond) N
Now, you must count the valence electrons to determine how many and where valence electrons are not forming bonds.
H has one valence electron which is in the H - C bond.
C has four valence electrons which are one in the H - C bond and three in the CN bonds.
N
has five valence electrons and it has three of them in the CN bonds,
therefore you have to add two electrons (points or other marks) to the N
atom.
This is the resultant Lewis structrue:
**
H - C (triple bond) N (draw the two ** close to the N symbol)
You have to draw three hyphens to represent the triple bond and include
the two ** over the N atom to represent the two electrons that are not
forming a bond.
