AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    The relative masses of protons, neutrons and electrons are: The sum of the…

    Full specification wording

    Size and mass of atoms: The relative masses of protons, neutrons and electrons are: The sum of the protons and neutrons in an atom is its mass number.

    Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible.

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 3 of 6
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible. The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. For example, a sodium atom with 11 protons and 12 neutrons has a mass number of 23. This value is always a whole number and is used to identify isotopes. The mass number is written as a superscript before the element symbol, such as ²³Na. Understanding mass number is essential for calculating relative atomic mass and interpreting isotopic abundance data.

    What earns the marks

    • State the relative masses: proton = 1, neutron = 1, electron ≈ 1/1836 (negligible).
    • Define mass number as the sum of protons and neutrons.
    • Calculate mass number using the formula: mass number = number of protons + number of neutrons.
    • Use the notation ᴬX where A is mass number and X is the element symbol.
    • Explain that mass number is always a whole number and is specific to an isotope.

    Common mistakes and how to put them right

    • Including electrons in the mass number calculation.

      Put it right by remembering that electrons have negligible mass and are not counted.

    • Confusing mass number with atomic number.

      Put it right: Atomic number is the number of protons, while mass number is protons plus neutrons.

    • Writing mass number as a subscript instead of a superscript.

      Put it right by placing it as a superscript before the symbol, e.g., ²³Na.

    Exam tips

    • Memorise the relative masses: proton = 1, neutron = 1, electron ≈ 0.
    • Use the formula mass number = protons + neutrons; rearrange to find neutrons if needed.
    • When writing symbols, put the mass number as a superscript on the left, e.g., ¹⁴C.

    Try the lesson

    The start of the Size and mass of atoms: The relative masses of protons, neutrons and electrons are: The sum of the protons and neutrons in an atom is its mass number. lesson, made from checked teaching notes for this specification point.

    Have a go first

    What is the mass number of an atom?

    Listen

    Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible. The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. For example, a sodium atom with 11 protons and 12 neutrons has a mass number of 23. This value is always a whole number and is used to identify isotopes. The mass number is written as a superscript before the element symbol, such as ²³Na. Understanding mass number is essential for calculating relative atomic mass and interpreting isotopic abundance data. A strong answer earns marks for these points. State the relative masses: proton = 1, neutron = 1, electron ≈ 1/1836 (negligible). Define mass number as the sum of protons and neutrons. Calculate mass number using the formula: mass number = number of protons + number of neutrons. Use the notation ᴬX where A is mass number and X is the element symbol. A common slip: Including electrons in the mass number calculation. Instead, remembering that electrons have negligible mass and are not counted. Exam tip: Memorise the relative masses: proton = 1, neutron = 1, electron ≈ 0.

    See it

    A top-down flowchart. Three nodes at the top give the relative masses: proton has relative mass 1, neutron has relative mass 1, and electron has relative mass about 1/1836, which is negligible. The proton and neutron nodes both lead down to a node reading 'Mass number = protons + neutrons'. The electron node leads separately to a node reading 'Electrons not counted in mass number'. From the mass number node, two arrows lead to two further nodes: 'Mass number is always a whole number' and 'Written as a superscript before the element symbol, e.g. ²³Na'.
    Shows the relative masses of protons, neutrons and electrons, and how protons and neutrons combine to give the mass number, which is written as a superscript before the element symbol.

    Key words

    Say what each word means before you turn the card.

    Relative mass of subatomic particles

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

    Mass number calculation

    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
    Relative mass of subatomic particles
    Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible.
    Mass number calculation
    The sum of the protons and neutrons in an atom is its mass number.
    Isotope notation
    The mass number is written as a superscript before the element symbol, such as ²³Na.

    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. Three nodes at the top give the relative masses: proton has relative mass 1, neutron has relative mass 1, and electron has relative mass about 1/1836, which is negligible. The proton and neutron nodes both lead down to a node reading 'Mass number = protons + neutrons'. The electron node leads separately to a node reading 'Electrons not counted in mass number'. From the mass number node, two arrows lead to two further nodes: 'Mass number is always a whole number' and 'Written as a superscript before the element symbol, e.g. ²³Na'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

    There is more to this lesson

    The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.

    Carry on with the full lesson

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Test your understanding of Size and mass of atoms: The relative masses of protons, neutrons and electrons are: The sum of the protons and neutrons in an atom is its mass number.

    Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

    Take topic quiz

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Start free

    Cookies on MasteryMind

    Essential cookies keep you signed in.

    Analytics and advertising cookies stay off unless you accept.

    Read our Privacy Policy.