Displayed formula

A displayed formula shows every atom in a molecule and every bond between them. Each atom is written as its symbol, and each covalent bond is drawn as a line: one line for a single bond, two lines for a double bond. For example, the displayed formula of methane, CH₄, is a C in the middle with four lines, each going to an H. Displayed formulas are used mostly for organic molecules, because they show exactly how the atoms are joined.

What is a displayed formula?

A molecular formula such as C₂H₆O tells you which atoms are in a molecule and how many, but not how they are joined. Two different molecules can share it. A displayed formula fixes that by drawing every bond. AQA's A-level Chemistry specification describes it as one of six ways of representing an organic compound, alongside empirical, molecular, general, structural and skeletal formulas, and asks students to draw structural, displayed and skeletal formulas for given compounds. At GCSE, AQA asks Chemistry students to draw fully displayed structural formulae of the first four alkenes and the products of their addition reactions.

In ethanol's displayed formula you can see straight away that the oxygen sits between a carbon and a hydrogen, as an O−H group. That is what makes it an alcohol.

The rules for drawing one

• Write every atom, including every hydrogen.
• Draw every bond as a line. A double bond is two parallel lines.
• Each atom must have its usual number of bonds:

A displayed formula is flat. It does not show the real 3D shape or the true bond angles, which is why carbon's four bonds are drawn at right angles even though they really point to the corners of a tetrahedron.

How to draw a displayed formula

1. Work out the carbon chain from the name. The stem gives the number of carbons: meth 1, eth 2, prop 3, but 4, pent 5, hex 6.
2. Draw the carbons in a row, joined by single bonds.
3. Add the functional group in the right place: a double bond for an alkene (-ene), O−H for an alcohol (-ol), the COOH group for a carboxylic acid (-oic acid).
4. Fill every spare bond on carbon with a hydrogen, so each carbon has exactly 4 bonds.
5. Check by counting the atoms against the molecular formula, and the bonds on every atom.

Displayed formulas of common molecules

Alkanes: methane and propane

Alkanes have only single bonds. Their general formula is CₙH₂ₙ₊₂.

Alkenes: ethene and propene

Alkenes have one carbon to carbon double bond. Their general formula is CₙH₂ₙ. Notice that the carbons in the double bond have only two other bonds each, so they carry fewer hydrogens.

In propene, the end carbon of the double bond has two hydrogens, the middle carbon has one, and the third carbon, joined by a single bond, has three. Counting: 3 carbons and 2 + 1 + 3 = 6 hydrogens, matching C₃H₆.

Alcohols: methanol and propan-1-ol

Alcohols contain the O−H group. The oxygen has two bonds, one to carbon and one to hydrogen.

In propan-1-ol the 1 means the O−H is on carbon 1, the end of the chain. Propan-2-ol has it on the middle carbon instead: the same atoms, joined differently.

Carboxylic acids: ethanoic acid and methanoic acid

Carboxylic acids contain the COOH group. Its carbon has a double bond to one oxygen and a single bond to an O−H. Writing COOH hides this, which is one reason exam questions ask for the displayed formula.

Esters: ethyl ethanoate

Esters contain a COO group linking two carbon chains. Ethyl ethanoate, CH₃COOCH₂CH₃, is made from ethanoic acid and ethanol:

    H   O       H   H
    |   ||      |   |
H - C - C - O - C - C - H
    |           |   |
    H           H   H

The two vertical lines above the second carbon are the C=O double bond. The oxygen in the middle has two single bonds, one to each side of the ester link.

From a displayed formula to a molecular formula

To get the molecular formula, count each type of atom in the drawing. For ethanoic acid above: 2 carbons, 4 hydrogens and 2 oxygens, so C₂H₄O₂. Count the hydrogens carefully: they are the easiest to miss, and the one on the oxygen counts too.

Now try matching some drawings to their molecular formulas, then press Mark to check them in the course.

Structural formula vs displayed formula

A structural formula gives the arrangement group by group, such as CH₃CH₂OH, with no bonds drawn. A displayed formula draws every atom and every bond. A skeletal formula goes the other way and leaves out the carbon and hydrogen atoms of the chain, drawing it as a zigzag line.

Use whichever the question asks for. If it says "displayed", every bond must be drawn: writing CH₃ or OH as a group will lose the mark.

Displayed formulas and isomers

Structural isomers have the same molecular formula but different displayed formulas. Propan-1-ol, drawn above, and propan-2-ol are both C₃H₈O:

Only the displayed formulas show the difference: in propan-1-ol the O−H is on an end carbon, and in propan-2-ol it is on the middle one. Butane and methylpropane, both C₄H₁₀, are another pair: one is a straight chain and the other has a branch. To check two drawings are really different, see whether one can be turned into the other just by bending or rotating the chain. If it can, they are the same molecule.

Common mistakes

• A carbon with five bonds, or three. Count the lines on every carbon: always 4.
• Missing the hydrogen on an O−H. Alcohols and carboxylic acids end in O−H, not just O.
• Writing groups instead of drawing them. CH₃ or COOH in a displayed formula is a structural formula, not a displayed one.
• Joining the O−H the wrong way round, as C−H−O. The oxygen bonds to the carbon; the hydrogen is on the end.
• One line for a double bond. Alkenes and the C=O in acids and esters need two lines.
• Counting a bent chain as a new molecule. A chain drawn with a bend is still a straight chain if every carbon is joined in a line.

Practise chemistry in a short timed paper

GCSE Chemistry Higher and A-level Chemistry: Year 1 each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so organic chemistry turns up regularly but not in every paper.

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