AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    The total number of protons and neutrons in an atom is called its mass…

    Full specification wording

    Mass number, atomic number and isotopes: The total number of protons and neutrons in an atom is called its mass number.

    The mass number of an atom is the sum of its protons and neutrons, because these are the only nucleons with appreciable mass.

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

    • Point 3 of 7
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The mass number of an atom is the sum of its protons and neutrons, because these are the only nucleons with appreciable mass. Electrons contribute almost nothing to the mass of an atom, so they are excluded. For example, a sodium atom with 11 protons and 12 neutrons has a mass number of 11 + 12 = 23. The mass number is always a whole number and is written as a superscript before the symbol, as in ²³Na. It is not the same as relative atomic mass, which is a weighted mean of isotope masses and is not usually a whole number. To find the number of neutrons, subtract the atomic number from the mass number: neutrons = mass number − atomic number.

    What earns the marks

    • Mass number equals the number of protons plus the number of neutrons in an atom.
    • Electrons are not included in the mass number because their mass is negligible compared with protons and neutrons.
    • The mass number is a whole number and is written as a superscript before the chemical symbol, for example ²³Na.
    • The number of neutrons can be calculated using: neutrons = mass number − atomic number.
    • Mass number is not the same as relative atomic mass, which is a weighted mean and may not be a whole number.

    Common mistakes and how to put them right

    • Including electrons in the mass number.

      Put it right: Only protons and neutrons are counted because electrons have negligible mass.

    • Confusing mass number with atomic number.

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

    • Writing the mass number as a subscript.

      Put it right: Mass number is written as a superscript before the symbol, for example ²³Na, while atomic number is written as a subscript.

    Exam tips

    • Read the question carefully to check whether it asks for mass number, atomic number or number of neutrons.
    • Show the addition or subtraction clearly, for example 11 + 12 = 23 or 23 − 11 = 12, so your method is visible.
    • Use the correct notation when writing nuclides, placing the mass number as a superscript before the symbol.

    Try the lesson

    The start of the Mass number, atomic number and isotopes: The total number of protons and neutrons in an atom is called its mass number. lesson, made from checked teaching notes for this specification point.

    Have a go first

    An atom of boron has 5 protons and 6 neutrons. What is the mass number of this atom?

    Listen

    The mass number of an atom is the sum of its protons and neutrons, because these are the only nucleons with appreciable mass. Electrons contribute almost nothing to the mass of an atom, so they are excluded. For example, a sodium atom with 11 protons and 12 neutrons has a mass number of 11 + 12 = 23. The mass number is always a whole number and is written as a superscript before the symbol, as in ²³Na. It is not the same as relative atomic mass, which is a weighted mean of isotope masses and is not usually a whole number. To find the number of neutrons, subtract the atomic number from the mass number: neutrons = mass number − atomic number. A strong answer earns marks for these points. Mass number equals the number of protons plus the number of neutrons in an atom. Electrons are not included in the mass number because their mass is negligible compared with protons and neutrons. The mass number is a whole number and is written as a superscript before the chemical symbol, for example ²³Na. The number of neutrons can be calculated using: neutrons = mass number − atomic number. A common slip: Including electrons in the mass number. Instead, only protons and neutrons are counted because electrons have negligible mass. Exam tip: Read the question carefully to check whether it asks for mass number, atomic number or number of neutrons.

    See it

    A flowchart. Two boxes, Protons and Neutrons, both lead into a central box reading Mass number = protons + neutrons. A box reading Electrons (negligible mass) has a dotted arrow labelled not counted pointing to the central box. From the central box, four arrows lead to separate boxes: Written as superscript before symbol, e.g. ²³Na; Neutrons = mass number − atomic number; Whole number; and Not the same as relative atomic mass.
    It shows that mass number is the sum of protons and neutrons, with electrons excluded, and the related points about notation, finding neutrons and relative atomic mass.

    Key words

    Say what each word means before you turn the card.

    Mass number

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

    Atomic number

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

    Relative atomic mass

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

    See every definition
    Mass number
    The total number of protons and neutrons in an atom.
    Atomic number
    The number of protons only, which is subtracted from the mass number to find the number of neutrons.
    Relative atomic mass
    A weighted mean of isotope masses that is not usually a whole number and is not the same as mass number.

    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. Two boxes, Protons and Neutrons, both lead into a central box reading Mass number = protons + neutrons. A box reading Electrons (negligible mass) has a dotted arrow labelled not counted pointing to the central box. From the central box, four arrows lead to separate boxes: Written as superscript before symbol, e.g. ²³Na; Neutrons = mass number − atomic number; Whole number; and Not the same as relative atomic mass.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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