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HONC if You Like Molecules: Drawing Structural Formulas
Chemistry in Context
The HONC 1234 Rule
The structural formulas for three molecules are shown here. Count the number of times each type of atom—
hydrogen, oxygen, nitrogen, and carbon—is connected to other atoms.
H
H C
H
H
C
H
C
H
H C H
H
N
H
H
C
H
H
C
H C H
H
H
C H
H

Every Hydrogen atom has one line connecting it to other atoms.

Every Oxygen atom has two lines connecting it to other atoms.

Every Nitrogen atom has three lines connecting it to other atoms.

Every Carbon atom has four lines connecting it to other atoms.
This information is sometimes referred to as the HONC 1234 rule. Within most molecules, hydrogen makes
one bond, oxygen makes two bonds, nitrogen makes three bonds, and carbon makes four bonds. The bonds in a
structural formula are represented by lines. One line connecting two atoms is called a single bond. A pair of
lines connecting the same two atoms, as in C=O, is called a double bond.
The HONC 1234 rule tells you how four of the most common nonmetal atoms will bond. You will learn about
the bonding of other nonmetal atoms like sulfur, S, and chlorine, Cl, in later lessons.
Drawing Structural Formulas
Example C3H8O: Consider a molecule with three carbon atoms, eight hydrogen atoms, and one oxygen atom.
Its molecular formula is C3H8O. What structural formula could this molecule have?
Start by connecting the carbon atoms.
CCC
Next insert the nitrogen or oxygen atoms, either on the ends or somewhere in the middle of the carbon chain.
CCCO
O
COCC
CCC
Add the hydrogen atoms last. Check that each atom follows the HONC 1234 rule. You must also check that
you haven’t drawn any duplicate structures. Three of the structures below are identical. Which are they?
H
O
H
H
H
C
C
C
H
H
H
a.
H
H
H
H
C
O
H
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b.
H
H
C
C
H
H
H
H
H
O
H
C
C
C
H
H
H
c.
H
H
H
H
H
H
C
C
C
H
H
H
O
H
H
H
H
C
C
C
H
H
O
e.
H
H
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d.
Questions
1. Use the HONC 1234 rule to create two unique (different) structural formulas for C2H6O.
2. Use the HONC 1234 rule to create three unique structural formulas for C3H9N.
3. The two molecular structures are incorrect according to the HONC 1234 rule. What specifically is wrong
with each? Correct them by drawing new structures.
H H H
COC
H H H
What’s wrong:
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H
H
CHOHC
H
H
What’s wrong:
2
Double and Triple Bonds
Many structural formulas have two atoms joined by two bonds; this type of bond is called a double bond.
Triple bonds are allowed, as well as ring structures (cyclic compounds); as long as they obey the HONC rule.
Examples: carbon dioxide, CO2
O=C=O
nitrogen, N2
cyclopentane
NN
4. Use the HONC 1234 rule to construct a structural formula (atoms and lines for
bonds) for the following. You will need to include one or more double bonds or
rings.
a. CH2O
b. C3H6
c. C4H6O
5. There are at least two molecules with the molecular formula C2H4O2. One is shown. Draw another one.
Hint: each molecule has one double bond; either between a carbon atom and one of the oxygen atoms,
C=O, or between two C atoms, C=C.
H
H
O
C
C
O
H
H
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Practice
1. Use the HONC 1234 rule to construct a structural formula for the following:
a. C2H7N
b. N2H4
c. C2H8N2
2. These molecules contain one or more double bonds or triple bonds or rings.
a. C3H4
b. C2H4O
c. C2H4O2
d. CHN
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