AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Mass number and isotopes: Mass number (A) and atomic (proton) number (Z).

    The atomic number Z is the number of protons in the nucleus, and it alone fixes which element you have; change it and you have a different element.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 1 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The atomic number Z is the number of protons in the nucleus, and it alone fixes which element you have; change it and you have a different element. The mass number A counts the protons and the neutrons together, so the number of neutrons is simply A − Z. In the standard notation the mass number sits above the atomic number to the left of the symbol. A is always a whole number because you cannot have a fraction of a nuclear particle, which is why a time of flight calculation that ends at 84.04 is reported as mass number 84. Relative atomic mass is a different quantity, an abundance-weighted mean across isotopes, and it is usually not a whole number.

    What earns the marks

    • The atomic number given as the number of protons.
    • The mass number given as the total number of protons and neutrons.
    • Neutrons obtained as mass number minus atomic number.
    • The mass number quoted as a whole number.

    Common mistakes and how to put them right

    • Quoting a relative atomic mass such as 186.3 as the mass number.

    • Leaving a calculated mass number as a decimal instead of rounding to the nearest whole number.

    • Swapping the positions of A and Z in the symbol notation.

    • Saying that atomic number changes between isotopes of the same element.

    Exam tips

    • Write A − Z under the symbol as soon as you meet it; the neutron count is needed more often than you expect.
    • If a calculation gives a non-integer mass number, round it and say so; examiners want a whole number.
    • Use the Periodic Table to check the atomic number matches the element named in the question.

    Try the lesson

    The start of the Mass number and isotopes: Mass number (A) and atomic (proton) number (Z). lesson, made from checked teaching notes for this specification point.

    Have a go first

    What does the atomic (proton) number of an atom represent?

    Listen

    The atomic number Z is the number of protons in the nucleus, and it alone fixes which element you have; change it and you have a different element. The mass number A counts the protons and the neutrons together, so the number of neutrons is simply A − Z. In the standard notation the mass number sits above the atomic number to the left of the symbol. A is always a whole number because you cannot have a fraction of a nuclear particle, which is why a time of flight calculation that ends at 84.04 is reported as mass number 84. Relative atomic mass is a different quantity, an abundance-weighted mean across isotopes, and it is usually not a whole number. A strong answer earns marks for these points. the atomic number given as the number of protons. the mass number given as the total number of protons and neutrons. neutrons obtained as mass number minus atomic number. the mass number quoted as a whole number. Exam tip: Write A − Z under the symbol as soon as you meet it; the neutron count is needed more often than you expect.

    See it

    A top-down flowchart. On the left, a node reading 'Atomic number Z = number of protons' points down to 'Fixes which element you have'. A separate node reading 'Mass number A = protons + neutrons' points down to 'Neutrons = A − Z', and the atomic number node also points to 'Neutrons = A − Z'. The mass number node additionally points to 'A is always a whole number'. On the right, a node reading 'Relative atomic mass' points down to 'Abundance-weighted mean across isotopes', which points down to 'Usually not a whole number'.
    It shows how atomic number and mass number relate to protons and neutrons, how neutrons are found as A − Z, and how relative atomic mass differs from mass number.

    Key words

    Say what each word means before you turn the card.

    atomic number

    Try answering aloud or in your head before turning the card.

    mass number

    Try answering aloud or in your head before turning the card.

    neutron count

    Try answering aloud or in your head before turning the card.

    isotope notation

    Try answering aloud or in your head before turning the card.

    See every definition
    atomic number
    The number of protons in the nucleus, and it alone fixes which element you have.
    mass number
    The total number of protons and neutrons, and it is always a whole number.
    neutron count
    Obtained as mass number minus atomic number.
    isotope notation
    The mass number sits above the atomic number to the left of the symbol.

    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. On the left, a node reading 'Atomic number Z = number of protons' points down to 'Fixes which element you have'. A separate node reading 'Mass number A = protons + neutrons' points down to 'Neutrons = A − Z', and the atomic number node also points to 'Neutrons = A − Z'. The mass number node additionally points to 'A is always a whole number'. On the right, a node reading 'Relative atomic mass' points down to 'Abundance-weighted mean across isotopes', which points down to 'Usually not a whole number'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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