AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Organic compounds can be represented by: empirical formula molecular…
Full specification wording
Nomenclature: Organic compounds can be represented by: empirical formula molecular formula general formula structural formula displayed formula skeletal formula.
Six ways of writing the same molecule, each showing different information.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Six ways of writing the same molecule, each showing different information. Take butane. Its empirical formula, the simplest whole number ratio, is C₂H₅. Its molecular formula, the actual numbers of atoms, is C₄H₁₀. The general formula for the alkane homologous series is CₙH₂ₙ₊₂. Its structural formula, CH₃CH₂CH₂CH₃, shows the arrangement without drawing every bond. Its displayed formula shows every atom and every bond, although bond angles need not be correct. Its skeletal formula is the plain zigzag, with carbons at the angles and ends and hydrogens on carbon left out. AQA insists that structures are unambiguous: C₃H₇Br will not do for 1-bromopropane, since it could equally be 2-bromopropane.
What earns the marks
- The empirical formula as the simplest whole number ratio of atoms.
- A displayed formula showing every atom and every bond in the molecule.
- A skeletal formula with carbon atoms shown by angles or intersections and all non-carbon atoms drawn in.
- An unambiguous structural formula, such as CH₃CH₂CH₂Br rather than C₃H₇Br.
Common mistakes and how to put them right
Giving a molecular formula such as C₃H₇Br where a structural formula was asked for, since it does not distinguish the isomers.
Omitting the hydrogen atoms of a functional group in a skeletal formula, when only hydrogens on carbon may be left out.
Drawing a displayed formula with an OH group shown as a single unit rather than as O bonded to H.
Drawing the bond from the carbon chain to the wrong atom of a nitrile, carboxylic acid or aldehyde group.
Exam tips
- Read which type of formula is asked for, because a skeletal drawing scores nothing where displayed was specified.
- In a displayed formula draw every C–H bond, even though the angles need not be accurate.
- Show the carbon to carbon bond clearly whenever the functional group itself contains a carbon atom.
Try the lesson
The start of the Nomenclature: Organic compounds can be represented by: empirical formula molecular formula general formula structural formula displayed formula skeletal formula. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of formula shows the simplest whole-number ratio of atoms of each element in an organic compound?
Listen
Six ways of writing the same molecule, each showing different information. Take butane. Its empirical formula, the simplest whole number ratio, is C₂H₅. Its molecular formula, the actual numbers of atoms, is C₄H₁₀. The general formula for the alkane homologous series is CₙH₂ₙ₊₂. Its structural formula, CH₃CH₂CH₂CH₃, shows the arrangement without drawing every bond. Its displayed formula shows every atom and every bond, although bond angles need not be correct. Its skeletal formula is the plain zigzag, with carbons at the angles and ends and hydrogens on carbon left out. AQA insists that structures are unambiguous: C₃H₇Br will not do for 1-bromopropane, since it could equally be 2-bromopropane. A strong answer earns marks for these points. the empirical formula as the simplest whole number ratio of atoms. a displayed formula showing every atom and every bond in the molecule. a skeletal formula with carbon atoms shown by angles or intersections and all non-carbon atoms drawn in. an unambiguous structural formula, such as CH₃CH₂CH₂Br rather than C₃H₇Br. Exam tip: Read which type of formula is asked for, because a skeletal drawing scores nothing where displayed was specified.
Key words
Say what each word means before you turn the card.
empirical formula
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molecular formula
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displayed formula
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skeletal formula
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See every definition
- empirical formula
- The simplest whole number ratio of atoms in a compound.
- molecular formula
- The actual numbers of atoms in a molecule.
- displayed formula
- A representation showing every atom and every bond, although bond angles need not be correct.
- skeletal formula
- A plain zigzag with carbons at the angles and ends and hydrogens on carbon left out.
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