AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Atoms can be represented as shown in this example:
Full specification wording
Mass number, atomic number and isotopes: Atoms can be represented as shown in this example:
Atoms are represented using nuclide notation, which shows the mass number as a superscript and the atomic number as a subscript before the chemical symbol.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 4 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Atoms are represented using nuclide notation, which shows the mass number as a superscript and the atomic number as a subscript before the chemical symbol. For example, ²³₁₁Na represents a sodium atom with mass number 23 and atomic number 11. The atomic number tells you the number of protons, which identifies the element. The mass number tells you the total number of protons and neutrons. From these two numbers you can work out the number of neutrons: neutrons = mass number − atomic number. In ²³₁₁Na, the number of neutrons is 23 − 11 = 12. This notation is used for all atoms and is essential for identifying isotopes, which have the same atomic number but different mass numbers.
What earns the marks
- Nuclide notation places the mass number as a superscript and the atomic number as a subscript before the chemical symbol.
- The atomic number equals the number of protons and determines the element.
- The mass number equals the number of protons plus the number of neutrons.
- The number of neutrons is calculated as mass number minus atomic number.
- Isotopes of an element have the same atomic number but different mass numbers.
Common mistakes and how to put them right
Swapping the positions of mass number and atomic number.
Put it right: Mass number is the superscript and atomic number is the subscript.
Thinking the atomic number equals the number of neutrons.
Put it right: Atomic number equals the number of protons; neutrons are found by subtracting atomic number from mass number.
Assuming all atoms of an element have the same mass number.
Put it right: Isotopes have different mass numbers, so the mass number must be stated for a particular nuclide.
Exam tips
- Write the symbol with the mass number clearly above and to the left, and the atomic number below and to the left.
- Check that the numbers you use are consistent with the element's identity: the atomic number must match the number of protons.
- When asked to identify an isotope, compare atomic numbers and mass numbers rather than just the element symbol.
Try the lesson
The start of the Mass number, atomic number and isotopes: Atoms can be represented as shown in this example: lesson, made from checked teaching notes for this specification point.
Have a go first
An atom of lithium is represented by the symbol ⁷₃Li. What does the number 3 in this symbol tell you about a lithium atom?
Listen
Atoms are represented using nuclide notation, which shows the mass number as a superscript and the atomic number as a subscript before the chemical symbol. For example, ²³₁₁Na represents a sodium atom with mass number 23 and atomic number 11. The atomic number tells you the number of protons, which identifies the element. The mass number tells you the total number of protons and neutrons. From these two numbers you can work out the number of neutrons: neutrons = mass number − atomic number. In ²³₁₁Na, the number of neutrons is 23 − 11 = 12. This notation is used for all atoms and is essential for identifying isotopes, which have the same atomic number but different mass numbers. A strong answer earns marks for these points. Nuclide notation places the mass number as a superscript and the atomic number as a subscript before the chemical symbol. The atomic number equals the number of protons and determines the element. The mass number equals the number of protons plus the number of neutrons. The number of neutrons is calculated as mass number minus atomic number. A common slip: Swapping the positions of mass number and atomic number. Instead, mass number is the superscript and atomic number is the subscript. Exam tip: Write the symbol with the mass number clearly above and to the left, and the atomic number below and to the left.
Key words
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Nuclide notation
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Atomic number
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Mass number
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Isotopes
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See every definition
- Nuclide notation
- A way of representing atoms that shows the mass number as a superscript and the atomic number as a subscript before the chemical symbol.
- Atomic number
- Tells you the number of protons, which identifies the element.
- Mass number
- Tells you the total number of protons and neutrons.
- Isotopes
- Atoms that have the same atomic number but different mass numbers.
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