Organic molecules can be written several different ways, and each one hides a bit more detail than the last. The skill isn’t memorising six definitions — it’s knowing which version a question is asking for, and being able to convert between them.
📚 What you need to know
Empirical formula — the simplest whole-number ratio of atoms.
Molecular formula — the actual number of each atom in one molecule.
Structural (displayed) formula — every atom and every bond drawn out.
Condensed structural formula — written on one line, with most bonds left out.
Skeletal formula — lines only; carbons sit at corners and ends, and C–H bonds are implied.
Stereochemical formula — uses wedges and dashes to show 3D arrangement.
The same molecule, five ways
The quickest way to see the difference is to write one molecule out in every format. Here is butane.
The further down this list you go, the more the drawing trusts you to fill in the obvious. All five describe butane.
Notice that nothing about the molecule changes as you move down that list — only how much you are choosing to draw. A skeletal formula isn’t a different molecule, it’s the same molecule with the obvious parts left off.
Empirical and molecular formulas
The molecular formula counts atoms. The empirical formula reduces that count to its simplest ratio, by dividing through by the highest common factor.
Sometimes they are identical. Butan-1-ol is C4H10O, and 4, 10 and 1 share no common factor, so the empirical formula is also C4H10O. But butane is C4H10, and both numbers divide by 2, giving C2H5.
WORKED EXAMPLE
Deduce the molecular and empirical formulas of ethanoic acid, CH3COOH, and of hexane.
Ethanoic acid — count every atom in the condensed formulaC: 1 + 1 = 2 H: 3 + 1 = 4 O: 2molecular = C₂H₄O₂All three divide by 2, so the ratio simplifies.empirical = CH₂OHexane — 6 carbons, so CₙH₂ₙ₊₂ gives 14 hydrogensmolecular = C₆H₁₄divide both by 2empirical = C₃H₇
Displayed and condensed formulas
A displayed (or structural, or graphical) formula shows every atom and every bond. It is the version to use when a question says “draw the structure” and wants to see that you know where the bonds are.
A condensed formula squeezes that onto one line. Atoms attached to a carbon are simply listed after it, and repeated units can be bracketed:
Pentane, condensed
CH3CH2CH2CH2CH3 or CH3(CH2)3CH3
Branches go in brackets straight after the carbon they are attached to. So CH3CH(CH3)CH2CH3 means a four-carbon chain with a methyl group hanging off the second carbon.
Double and triple bonds are the exception — they are always shown, because you could not work them out otherwise. Propene is written CH2=CHCH3.
Counting hydrogens is the check. Every carbon must end up with four bonds. If a carbon in a condensed formula seems to have only three, there is a double bond you have missed.
Skeletal formulas
Skeletal formulas look almost empty at first, which is exactly why they are so useful for bigger molecules — the functional groups stand out instead of drowning in hydrogens.
Skeletal formulas hide the hydrogens on carbon so the functional group is the first thing you see.
🧩 The three rules
Carbon–carbon bonds are drawn as plain lines in a zig-zag.
Every corner and every free end is a carbon atom. They are not labelled.
Hydrogens attached to carbon are not drawn. Everything else — O, N, halogens, and any H inside a functional group like –OH — is drawn.
One consequence catches people out: methane has no skeletal formula. Skeletal drawings are built from carbon–carbon bonds, and methane only has one carbon, so there is nothing to draw. A single dot would be read as a lone atom or a radical.
WORKED EXAMPLE
A skeletal formula shows a chain of five plain line segments, with an OH written on the last vertex. Deduce the molecular formula and name the compound.
Count the carbons — corners and ends onlyFive segments, with the last one going to an O, so there are 5 carbon vertices.Fill in the hidden hydrogens so every carbon has 4 bondsCH₃ – CH₂ – CH₂ – CH₂ – CH₂ – OHC₅H₁₂O, pentan–1–ol
Stereochemical formulas
Sometimes flat isn’t enough. A stereochemical formula shows the actual three-dimensional arrangement of bonds around a carbon, which matters when two molecules have the same connections but are mirror images of each other.
Two bonds stay in the plane of the paper, one comes towards you and one goes behind, giving the tetrahedral arrangement its 109.5 degree angles.
Plain line — the bond lies in the plane of the paper.
Solid wedge — the bond points out of the paper, towards you.
Dashed wedge — the bond points behind the paper, away from you.
A carbon with four different groups attached is tetrahedral, with bond angles of about 109.5°, because the four bonding pairs repel each other as far apart as possible in three dimensions.
💡 Exam tip
Read the command word. “Displayed formula” means draw every bond, including all the C–H ones. Losing those loses marks.
When converting from skeletal, add the hydrogens carbon by carbon so that each has four bonds.
Empirical formula questions are just dividing by a common factor. Check whether one exists before answering.
Keep bond angles roughly realistic when drawing skeletal chains — a zig-zag, not a straight line.
⚠️ Common mix-up
Empirical is not the same as molecular, but sometimes they happen to be identical. Check, don’t assume either way.
Skeletal formulas do hide hydrogens on carbon — but never the H in –OH, –COOH or –NH2.
Double bonds always appear in a condensed formula. CH3CH2CH3 and CH2=CHCH3 are different compounds.
Methane has no skeletal formula. Don’t draw a dot.
A wedge is not just decoration — solid means towards you, dashed means away.
Up next: Functional Groups — the small clusters of atoms that decide what a molecule actually does, no matter how long its carbon chain is.
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