AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Fractional distillation and petrochemicals: The vast array of natural and synthetic carbon compounds occur due to the ability of carbon atoms to form families of similar compounds.

    Carbon has four electrons in its outer shell, so each carbon atom can form four covalent bonds.

    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Carbon has four electrons in its outer shell, so each carbon atom can form four covalent bonds. This lets carbon atoms bond to one another in chains, branches and rings, and also bond to hydrogen, oxygen, nitrogen and halogens. Because the same small set of bonding patterns repeats, carbon compounds fall into families called homologous series, such as alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH) and carboxylic acids (CₙH₂ₙ₊₁COOH). Natural compounds come from living things, for example the hydrocarbons in crude oil and the carbohydrates in plants; synthetic compounds are made by humans, for example plastics such as poly(ethene) and medicines such as aspirin. Millions of different compounds are possible because the chain length, branching and functional group can all vary.

    What earns the marks

    • Carbon atoms each form four covalent bonds, which allows carbon to bond to other carbon atoms and to elements such as hydrogen, oxygen, nitrogen and chlorine.
    • Carbon atoms can join into chains of different lengths, and the chains may be straight, branched or arranged in rings.
    • A homologous series is a family of compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH₂ unit.
    • Natural carbon compounds are obtained from living organisms or geological sources, for example hydrocarbons in crude oil and carbohydrates in plants.
    • Synthetic carbon compounds are manufactured by humans, for example poly(ethene) from ethene and aspirin from salicylic acid.
    • The variety of possible chain lengths, branching patterns and functional groups explains why so many different carbon compounds exist.

    Common mistakes and how to put them right

    • Thinking carbon forms only one bond.

      Put it right: Carbon has four outer electrons, so it forms four covalent bonds, as in methane CH₄.

    • Confusing natural with synthetic.

      Put it right: Natural compounds are produced by living things or found in nature, whereas synthetic compounds are made by humans, even if they copy a natural substance.

    • Assuming a homologous series contains identical compounds.

      Put it right: Members share a general formula and similar reactions but have different numbers of carbon atoms, so their physical properties change gradually.

    Exam tips

    • When asked why carbon forms so many compounds, link the answer to carbon's four covalent bonds and its ability to form chains, branches and rings.
    • Give one named natural example and one named synthetic example, and state clearly which is which.
    • If a general formula is given, check it against a named member, for example C₃H₈ fits CₙH₂ₙ₊₂ for an alkane.

    Try the lesson

    The start of the Fractional distillation and petrochemicals: The vast array of natural and synthetic carbon compounds occur due to the ability of carbon atoms to form families of similar compounds. lesson, made from checked teaching notes for this specification point.

    Have a go first

    There is a vast array of natural and synthetic carbon compounds. What property of carbon atoms allows so many different compounds to exist?

    Listen

    Carbon has four electrons in its outer shell, so each carbon atom can form four covalent bonds. This lets carbon atoms bond to one another in chains, branches and rings, and also bond to hydrogen, oxygen, nitrogen and halogens. Because the same small set of bonding patterns repeats, carbon compounds fall into families called homologous series, such as alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH) and carboxylic acids (CₙH₂ₙ₊₁COOH). Natural compounds come from living things, for example the hydrocarbons in crude oil and the carbohydrates in plants; synthetic compounds are made by humans, for example plastics such as poly(ethene) and medicines such as aspirin. Millions of different compounds are possible because the chain length, branching and functional group can all vary. A strong answer earns marks for these points. Carbon atoms each form four covalent bonds, which allows carbon to bond to other carbon atoms and to elements such as hydrogen, oxygen, nitrogen and chlorine. Carbon atoms can join into chains of different lengths, and the chains may be straight, branched or arranged in rings. A homologous series is a family of compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH₂ unit. Natural carbon compounds are obtained from living organisms or geological sources, for example hydrocarbons in crude oil and carbohydrates in plants. Exam tip: When asked why carbon forms so many compounds, link the answer to carbon's four covalent bonds and its ability to form chains, branches and rings.

    See it

    A top-down flowchart. The first box says carbon has four outer electrons and points to a box saying each carbon atom forms four covalent bonds. That box leads to two boxes: carbon bonds to other carbon atoms, and carbon bonds to hydrogen, oxygen, nitrogen and halogens. The first of these leads to three boxes: chains of different lengths, branches, and rings. All three of these lead to a box saying families of similar compounds (homologous series), which leads to a final box saying millions of different carbon compounds.
    It shows how carbon's four covalent bonds allow chains, branches and rings, leading to homologous series and millions of carbon compounds.

    Key words

    Say what each word means before you turn the card.

    Homologous series

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    Natural carbon compounds

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    Synthetic carbon compounds

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    See every definition
    Homologous series
    A family of compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH₂ unit.
    Natural carbon compounds
    Compounds obtained from living organisms or geological sources, for example hydrocarbons in crude oil and carbohydrates in plants.
    Synthetic carbon compounds
    Compounds manufactured by humans, for example poly(ethene) from ethene and aspirin from salicylic acid.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box says carbon has four outer electrons and points to a box saying each carbon atom forms four covalent bonds. That box leads to two boxes: carbon bonds to other carbon atoms, and carbon bonds to hydrogen, oxygen, nitrogen and halogens. The first of these leads to three boxes: chains of different lengths, branches, and rings. All three of these lead to a box saying families of similar compounds (homologous series), which leads to a final box saying millions of different carbon compounds.

    • 1Box 1

    • 2Box 2

    • 3Box 3

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