Lewis structure of HCN
10 valence electrons. One C-H single bond and one C≡N triple bond, with no lone pair on C.
Lewis structure
10 valence electronsFormal charges: none
Why this shape
- C has 4 valence electrons. It uses four in one single bond to H and one triple bond to N, so none are left over.
- Steric number = two sigma bonds + no lone pairs = 2, so C is sp hybridised. A double or triple bond counts once: only its sigma bond uses a hybrid orbital.
- Two electron pairs point in opposite directions, along a line (linear electron geometry). With no lone pairs the atoms take the same shape: linear.
- All the bond pairs repel each other equally, so the angle is the ideal 180°.
- The bond dipoles do not cancel in the linear shape, so HCN is polar.
How to draw it
- Count the valence electrons: C 4 + H 1 + N 5 = 10 valence electrons, 5 pairs.
- Put C in the middle and join it to the two outer atoms with single bonds.
- Give each outer atom lone pairs until it has an octet (H needs only two electrons), then put any electrons left over on the central atom.
- Where an atom is still short of an octet, turn a lone pair of its neighbour into a shared pair: that makes a double or triple bond.
- Result: one C-H single bond and one C≡N triple bond, with no lone pair on C.
- Formal charge = valence electrons − lone pair electrons − number of bonds. Here: none.
Class 11, Chapter 4 Chemical Bonding and Molecular Structure.
Lewis structure of HCN: quick answers
How many valence electrons are in HCN?
C 4 + H 1 + N 5 = 10 valence electrons, 5 pairs.
What are the formal charges in the Lewis structure of HCN?
Every atom has a formal charge of zero.
How many lone pairs are in the Lewis structure of HCN?
1 in all, none on the central C.