AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Atoms of the same element can have different numbers of neutrons; these…
Full specification wording
Size and mass of atoms: Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
Isotopes are atoms of the same element that have the same number of protons (and therefore the same atomic number) but different numbers of neutrons.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 6
Study notes
From the checked teaching notes for this specification point.
The idea
Isotopes are atoms of the same element that have the same number of protons (and therefore the same atomic number) but different numbers of neutrons. This means they have different mass numbers. For example, chlorine has two common isotopes: chlorine-35 (17 protons, 18 neutrons) and chlorine-37 (17 protons, 19 neutrons). Because isotopes have the same electron configuration, they have identical chemical properties, but their physical properties may differ slightly due to different masses. Isotopes are represented using symbols like ³⁵Cl and ³⁷Cl. Understanding isotopes is essential for explaining relative atomic mass and radioactive decay.
What earns the marks
- Define isotopes as atoms of the same element with different numbers of neutrons.
- State that isotopes have the same number of protons and electrons but different numbers of neutrons.
- Give examples of isotopes, such as chlorine-35 and chlorine-37, or carbon-12 and carbon-14.
- Explain that isotopes have the same chemical properties but different physical properties.
- Calculate the relative atomic mass from isotopic abundances.
Common mistakes and how to put them right
Thinking that isotopes have different numbers of protons.
Put it right by emphasising that protons define the element and must be the same.
Confusing isotopes with ions.
Put it right: Ions have different numbers of electrons, while isotopes have different numbers of neutrons.
Assuming isotopes have different chemical properties.
Put it right by explaining that chemical properties depend on electron arrangement, which is the same.
Exam tips
- Remember that isotopes of an element have the same atomic number but different mass numbers.
- Use the format 'element-mass number' (e.g., carbon-14) or symbol with superscript (e.g., ¹⁴C).
- When calculating relative atomic mass, use the formula: (abundance₁ × mass₁ + abundance₂ × mass₂) / total abundance.
Try the lesson
The start of the Size and mass of atoms: Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element. lesson, made from checked teaching notes for this specification point.
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What are isotopes? Select the correct answer.
Listen
Isotopes are atoms of the same element that have the same number of protons (and therefore the same atomic number) but different numbers of neutrons. This means they have different mass numbers. For example, chlorine has two common isotopes: chlorine-35 (17 protons, 18 neutrons) and chlorine-37 (17 protons, 19 neutrons). Because isotopes have the same electron configuration, they have identical chemical properties, but their physical properties may differ slightly due to different masses. Isotopes are represented using symbols like ³⁵Cl and ³⁷Cl. Understanding isotopes is essential for explaining relative atomic mass and radioactive decay. A strong answer earns marks for these points. Define isotopes as atoms of the same element with different numbers of neutrons. State that isotopes have the same number of protons and electrons but different numbers of neutrons. Give examples of isotopes, such as chlorine-35 and chlorine-37, or carbon-12 and carbon-14. Explain that isotopes have the same chemical properties but different physical properties. A common slip: Thinking that isotopes have different numbers of protons. Instead, emphasising that protons define the element and must be the same. Exam tip: Remember that isotopes of an element have the same atomic number but different mass numbers.
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