AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Students should be able to relate differences between isotopes to…

    Full specification wording

    Mass number, atomic number and isotopes: Students should be able to relate differences between isotopes to differences in conventional representations of their identities, charges and masses.

    Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

    Checked against the AQA 8464 specification (opens in a new tab) on .

    • Point 7 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. In conventional representations, the identity is the element symbol, the charge is shown as a superscript, and the mass is shown as a superscript mass number. For example, chlorine-35 and chlorine-37 are represented as ³⁵Cl and ³⁷Cl. Both have atomic number 17, so 17 protons and 17 electrons in a neutral atom, giving no overall charge. The mass number differs because the neutron number differs: 18 neutrons in ³⁵Cl and 20 neutrons in ³⁷Cl. If an isotope forms an ion, the charge is shown separately, such as ³⁵Cl⁻, which has 18 electrons and an overall charge of 1⁻. Thus differences in isotope identity, charge and mass are reflected in the mass number, charge superscript and element symbol.

    What earns the marks

    • Isotopes have the same number of protons but different numbers of neutrons, so their mass numbers differ.
    • The conventional representation includes the element symbol, a mass number superscript and, for ions, a charge superscript.
    • The mass number equals the number of protons plus the number of neutrons, so it distinguishes isotopes of the same element.
    • The charge is determined by the difference between the number of protons and the number of electrons, not by the number of neutrons.
    • For a neutral atom, the number of electrons equals the number of protons, so the overall charge is zero.
    • For an ion, the charge superscript shows the overall charge, for example 1⁺, 2⁺, 1⁻ or 2⁻.

    Common mistakes and how to put them right

    • Confusing mass number with relative atomic mass.

      Put it right: Mass number is the total number of protons and neutrons in a specific isotope, while relative atomic mass is a weighted average for the element.

    • Thinking that isotopes have different chemical identities because they have different masses.

      Put it right: Isotopes of the same element have the same atomic number and the same number of protons, so they are the same element.

    • Writing the charge as a mass number or omitting the charge superscript for ions.

      Put it right: Use a separate superscript for charge, such as ³⁵Cl⁻, and a superscript for mass number, such as ³⁵Cl.

    Exam tips

    • When interpreting a representation such as ³⁷Cl⁻, state the number of protons, neutrons and electrons before explaining the charge and mass.
    • Use the periodic table to check the atomic number, then calculate neutrons as mass number minus atomic number.
    • For comparison questions, refer to both isotopes explicitly and link each difference to the representation, for example the mass number superscript or the charge superscript.

    Try the lesson

    The start of the Mass number, atomic number and isotopes: Students should be able to relate differences between isotopes to differences in conventional representations of their identities, charges and masses. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Two atoms of chlorine are represented as ³⁵Cl and ³⁷Cl. Which statement describes why these two representations show isotopes of chlorine?

    Listen

    Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. In conventional representations, the identity is the element symbol, the charge is shown as a superscript, and the mass is shown as a superscript mass number. For example, chlorine-35 and chlorine-37 are represented as ³⁵Cl and ³⁷Cl. Both have atomic number 17, so 17 protons and 17 electrons in a neutral atom, giving no overall charge. The mass number differs because the neutron number differs: 18 neutrons in ³⁵Cl and 20 neutrons in ³⁷Cl. If an isotope forms an ion, the charge is shown separately, such as ³⁵Cl⁻, which has 18 electrons and an overall charge of 1⁻. Thus differences in isotope identity, charge and mass are reflected in the mass number, charge superscript and element symbol. A strong answer earns marks for these points. Isotopes have the same number of protons but different numbers of neutrons, so their mass numbers differ. The conventional representation includes the element symbol, a mass number superscript and, for ions, a charge superscript. The mass number equals the number of protons plus the number of neutrons, so it distinguishes isotopes of the same element. The charge is determined by the difference between the number of protons and the number of electrons, not by the number of neutrons. A common slip: Confusing mass number with relative atomic mass. Instead, mass number is the total number of protons and neutrons in a specific isotope, while relative atomic mass is a weighted average for the element. Exam tip: When interpreting a representation such as ³⁷Cl⁻, state the number of protons, neutrons and electrons before explaining the charge and mass.

    See it

    A flowchart starting with a box for the conventional representation, which contains the element symbol, the mass number superscript and the charge superscript. Three arrows lead from it: one to identity, the element symbol; one to mass, the mass number superscript; and one to charge, the charge superscript. From mass, an arrow leads to mass number equals protons plus neutrons, then to same protons but different neutrons, then to different mass numbers, ³⁵Cl and ³⁷Cl. From charge, an arrow leads to charge from protons minus electrons, which branches to a neutral atom where electrons equal protons and charge is zero, and to an ion where the charge is shown separately, for example ³⁵Cl⁻.
    It shows how the parts of a conventional representation relate to identity, mass number and charge, and how differing neutrons give different mass numbers for isotopes.

    Key words

    Say what each word means before you turn the card.

    Isotopes

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    Conventional representation

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    Mass number

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    Charge

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    See every definition
    Isotopes
    Atoms of the same element with the same number of protons but different numbers of neutrons.
    Conventional representation
    Shows the identity as the element symbol, the charge as a superscript, and the mass as a superscript mass number.
    Mass number
    Equals the number of protons plus the number of neutrons, so it distinguishes isotopes of the same element.
    Charge
    Determined by the difference between the number of protons and the number of electrons, not by the number of neutrons.

    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 flowchart starting with a box for the conventional representation, which contains the element symbol, the mass number superscript and the charge superscript. Three arrows lead from it: one to identity, the element symbol; one to mass, the mass number superscript; and one to charge, the charge superscript. From mass, an arrow leads to mass number equals protons plus neutrons, then to same protons but different neutrons, then to different mass numbers, ³⁵Cl and ³⁷Cl. From charge, an arrow leads to charge from protons minus electrons, which branches to a neutral atom where electrons equal protons and charge is zero, and to an ion where the charge is shown separately, for example ³⁵Cl⁻.

    • 1Box 1

    • 2Box 2

    • 3Box 3

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