AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Z Meaning of isotopes and the use of isotopic data.

    Full specification wording

    Constituents of the atom: Z Meaning of isotopes and the use of isotopic data.

    Isotopes are atoms of the same element with the same proton number Z but different nucleon numbers A, so they have different numbers of neutrons.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 5 of 5
    • 3 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 proton number Z but different nucleon numbers A, so they have different numbers of neutrons. For example, ¹²₆C and ¹⁴₆C both have Z = 6, but ¹²₆C has 6 neutrons and ¹⁴₆C has 8 neutrons. Because chemical properties depend mainly on electron configuration, isotopes of an element have similar chemical behaviour, but their physical properties and nuclear stability can differ. Isotopic data include relative abundances and masses, which are used to calculate relative atomic mass. For an element with isotopes, the relative atomic mass is the abundance-weighted mean of the isotope masses, so a sample’s isotopic composition determines its measured relative atomic mass.

    What earns the marks

    • Isotopes have the same proton number Z but different nucleon numbers A.
    • Isotopes of an element have different numbers of neutrons, calculated as A − Z.
    • Isotopes have the same number of electrons in a neutral atom, so their chemical properties are similar.
    • Isotopic data include relative abundances and isotopic masses.
    • Relative atomic mass is calculated as the sum of (isotope abundance × isotope mass) divided by the total abundance, using consistent abundance units.
    • Isotopic data can be used to identify an element or confirm a sample’s composition from its measured relative atomic mass.

    Common mistakes and how to put them right

    • Defining isotopes as atoms with different proton numbers.

      Put it right: Isotopes have the same Z but different A.

    • Using unweighted averages of isotope masses.

      Put it right: Weight each isotope mass by its abundance.

    • Assuming isotopes have identical physical properties.

      Put it right: Physical properties and nuclear stability can differ.

    Exam tips

    • When calculating relative atomic mass, convert percentage abundances to fractions or use the percentage values consistently in the weighted sum.
    • Show the weighted sum expression before evaluating it so the method is clear.
    • Link the number of neutrons to A − Z for each isotope to avoid mixing up mass number and neutron number.

    Try the lesson

    The start of the Constituents of the atom: Z Meaning of isotopes and the use of isotopic data. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which statement describes isotopes of the same element?

    Listen

    Isotopes are atoms of the same element with the same proton number Z but different nucleon numbers A, so they have different numbers of neutrons. For example, ¹²₆C and ¹⁴₆C both have Z = 6, but ¹²₆C has 6 neutrons and ¹⁴₆C has 8 neutrons. Because chemical properties depend mainly on electron configuration, isotopes of an element have similar chemical behaviour, but their physical properties and nuclear stability can differ. Isotopic data include relative abundances and masses, which are used to calculate relative atomic mass. For an element with isotopes, the relative atomic mass is the abundance-weighted mean of the isotope masses, so a sample’s isotopic composition determines its measured relative atomic mass. A strong answer earns marks for these points. Isotopes have the same proton number Z but different nucleon numbers A. Isotopes of an element have different numbers of neutrons, calculated as A − Z. Isotopes have the same number of electrons in a neutral atom, so their chemical properties are similar. Isotopic data include relative abundances and isotopic masses. A common slip: Defining isotopes as atoms with different proton numbers: the error is changing Z. Instead, isotopes have the same Z but different A. Exam tip: When calculating relative atomic mass, convert percentage abundances to fractions or use the percentage values consistently in the weighted sum.

    See it

    A top-down flowchart. The top node reads 'Isotopes: same proton number Z, different nucleon number A'. Four arrows lead from it. The first goes to 'Different number of neutrons = A − Z'. The second goes to 'Same number of electrons in a neutral atom', which then leads to 'Similar chemical properties'. The third goes to 'Physical properties and nuclear stability can differ'. The fourth goes to 'Isotopic data: relative abundances and isotopic masses', which branches into two nodes: 'Relative atomic mass = sum of (abundance × isotope mass) divided by total abundance' and 'Identify an element or confirm a sample's composition from measured relative atomic mass'.
    It shows how the definition of isotopes leads to their differing neutrons, similar chemical behaviour, differing physical properties, and the use of isotopic data to calculate relative atomic mass and identify an element.

    Key words

    Say what each word means before you turn the card.

    Isotopes

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    Relative atomic mass

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    Isotopic data

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    See every definition
    Isotopes
    Atoms of the same element with the same proton number Z but different nucleon numbers A, so they have different numbers of neutrons.
    Relative atomic mass
    The abundance-weighted mean of the isotope masses, calculated as the sum of (isotope abundance × isotope mass) divided by the total abundance.
    Isotopic data
    Relative abundances and masses, which are used to calculate relative atomic mass.

    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 top node reads 'Isotopes: same proton number Z, different nucleon number A'. Four arrows lead from it. The first goes to 'Different number of neutrons = A − Z'. The second goes to 'Same number of electrons in a neutral atom', which then leads to 'Similar chemical properties'. The third goes to 'Physical properties and nuclear stability can differ'. The fourth goes to 'Isotopic data: relative abundances and isotopic masses', which branches into two nodes: 'Relative atomic mass = sum of (abundance × isotope mass) divided by total abundance' and 'Identify an element or confirm a sample's composition from measured relative atomic mass'.

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    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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