In a homologous series, successive members differ by which unit?

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Study for the IGCSE Edexcel Chemistry Test. Access flashcards and multiple choice questions with hints and explanations. Get ready for your exam!

Multiple Choice

In a homologous series, successive members differ by which unit?

Explanation:
In a homologous series, successively related compounds grow by adding the same small fragment to the carbon chain. That fragment is CH2, the methylene group. For alkanes the pattern is CnH2n+2, so going from one member to the next adds one carbon and two hydrogens, which is exactly CH2. For example, methane (CH4) becomes ethane (C2H6) by adding CH2, and ethane becomes propane (C3H8) by adding another CH2. The other options don’t fit because adding CH3 would change the hydrogen count differently and would break the consistent formula pattern, while CO2 and H2O are not the repeating unit used to extend the hydrocarbon chain.

In a homologous series, successively related compounds grow by adding the same small fragment to the carbon chain. That fragment is CH2, the methylene group. For alkanes the pattern is CnH2n+2, so going from one member to the next adds one carbon and two hydrogens, which is exactly CH2. For example, methane (CH4) becomes ethane (C2H6) by adding CH2, and ethane becomes propane (C3H8) by adding another CH2. The other options don’t fit because adding CH3 would change the hydrogen count differently and would break the consistent formula pattern, while CO2 and H2O are not the repeating unit used to extend the hydrocarbon chain.

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