To draw a small molecule correctly, give every atom its usual number of bonds: carbon four, oxygen two, hydrogen one. Draw each bond as a line, and count the lines on each atom before you finish.
This skill builds on identifying a functional group and on the covalent bonding in bonding and structure. It is used whenever a question says “draw the displayed formula”.
What rules does a correct drawing follow?
- Each carbon has exactly 4 bonds, counting a double bond as 2.
- Each hydrogen has exactly 1 bond.
- Each oxygen has exactly 2 bonds, either two single bonds or one double bond.
- A displayed formula shows every atom and every bond. No atom is left bare, and no C-H bond is skipped.
A drawing that obeys these numbers will match a real molecule. One that does not cannot be correct, whatever the atoms look like.
How do you build a drawing, step by step?
- Write the carbon skeleton as a chain of C atoms joined by single bonds.
- Add the functional group in the right place: a C=C, an -OH or a -COOH.
- Fill every carbon’s remaining bonds with hydrogen until each has four.
- Check oxygen has two bonds and every hydrogen has one.
- Count total bonds if you need to double-check against the formula.
Worked example
Draw the displayed formula of ethanol, C₂H₆O.
Skeleton. Two carbons joined: C-C. Functional group. Put an -OH on one carbon: C-C-O-H.
Fill with hydrogens. The left carbon has one bond (to the other carbon), so it needs three hydrogens. The right carbon already has two bonds (to C and to O), so it needs two hydrogens.
H H
| |
H - C - C - O - H
| |
H HCheck. Left carbon: three H plus one C gives 4. Right carbon: two H, one C and one O gives 4. Oxygen: one C and one H gives 2. Every hydrogen has one bond. Counting atoms gives 2 C, 5 + 1 = 6 H and 1 O, matching C₂H₆O.
Here is ethene, C₂H₄, for contrast. The double bond uses two of each carbon’s four bonds, so each carbon takes only two hydrogens.
H H
\ /
C = C
/ \
H HEach carbon has 2 H plus a double bond (counts 2) and so has 4 bonds. That matches C₂H₄.
The mistake to watch for
The typical slip is to draw a carbon with five bonds, often by adding a hydrogen to a carbon that already has a double bond.
Mistaken drawing: a carbon with a double bond to the other carbon and three hydrogens.
That carbon has 2 + 3 = 5 bonds. No carbon can have more than four.
Count each carbon before you finish. If it has five, remove a hydrogen. If it has three, a hydrogen is missing.
The same counting catches oxygen atoms drawn with one bond or three.
Check yourself
1. Draw the displayed formula of methanol, CH₃OH, and count the bonds on the carbon.
Show answer
H
|
H - C - O - H
|
HThe carbon has three C-H bonds and one C-O bond, so 4 bonds. Oxygen has one C-O and one O-H bond, so 2 bonds.
2. How many covalent bonds are in one molecule of ethane, C₂H₆?
Show answer
One C-C bond and six C-H bonds gives 7 bonds.
3. A student draws a carbon atom with a C=C double bond, a C-C single bond and two C-H bonds. Is this allowed? Explain.
Show answer
No. The double bond counts as 2, the single bond as 1 and the two C-H bonds as 2, so the carbon has 5 bonds. A carbon forms four bonds, so one C-H bond must be removed.
Where this leads next
Drawing correctly prepares you to tell molecules apart by double bonds in recognising saturated and unsaturated structures. The equation balance reasoning trainer gives quick atom-counting practice that uses the same habit.
If your drawings are right when you take your time but go wrong under pressure, online one-to-one Chemistry tuition can build a fast self-check routine with you.