8.6 - Homologous series
Recognise the formula pattern in a homologous series and distinguish changing physical properties from similar chemical behaviour.
A family with a formula pattern
A homologous series is a family of compounds sharing a general formula. Neighbouring members differ by CH₂; their physical properties vary gradually and their chemical properties are similar.
A general formula gives a rule for the whole family. Alkanes have the general formula CₙH₂ₙ₊₂, where n is the number of carbon atoms.
| n | Molecular formula |
|---|---|
| 1 | CH₄ |
| 2 | C₂H₆ |
| 3 | C₃H₈ |
| 4 | C₄H₁₀ |
For n = 4, hydrogen atoms = 2 × 4 + 2 = 10, giving C₄H₁₀. The molecular formulae differ, even though they obey the same general formula.
What a neighbouring step means
Going from C₂H₆ to C₃H₈ adds one carbon and two hydrogen atoms to the formula: a difference of CH₂. This compares formulae; it does not describe adding a free CH₂ molecule in a reaction.
CH₄ and C₃H₈ are in the same series but are not neighbours. They are two CH₂ steps apart.
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