AQA · GCSE · Combined Science

Course version: 8464 · Version from Sept 2018

Size and mass of atoms: Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.

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Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they have the same atomic number but different mass numbers.

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Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Isotopes have the same number of protons, so they are atoms of the same element.
Isotopes have different numbers of neutrons and therefore different mass numbers.
The atomic number is the same for all isotopes of an element.
Chemical properties are essentially the same because electron arrangement is the same.
Relative atomic mass is a weighted mean of the masses of the isotopes, so it may not be a whole number.
Isotopes of an element may include stable and radioactive forms.

Common mistakes and how to put them right

Saying isotopes have different numbers of protons.
Put it right by stating that proton number is identical and only neutron number differs.
Claiming isotopes have completely different chemical properties.
Put it right by explaining that identical electron arrangements give essentially the same chemistry.
Treating relative atomic mass as a simple average of mass numbers.
Put it right by using the weighted mean that accounts for isotopic abundance.

Exam tips

Define isotopes using both 'same protons' and 'different neutrons' to secure the mark.
Use a named example such as chlorine-35 and chlorine-37 to make the explanation concrete.
If asked to compare, state clearly what is the same and what is different rather than describing each isotope separately.

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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Atoms of the same element that have different numbers of neutrons are called what?

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Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they have the same atomic number but different mass numbers. Chlorine, for example, has isotopes with mass numbers 35 and 37 (³⁵Cl and ³⁷Cl): both have 17 protons, but one has 18 neutrons and the other 20. Because proton number fixes the element, isotopes share chemical properties; differing neutron numbers change mass and some physical behaviour. Relative atomic mass is the weighted mean of isotope masses, so it need not be a whole number. Isotopes may be stable or radioactive. A strong answer earns marks for these points. Isotopes have the same number of protons, so they are atoms of the same element. Isotopes have different numbers of neutrons and therefore different mass numbers. The atomic number is the same for all isotopes of an element. Chemical properties are essentially the same because electron arrangement is the same. A common slip: Saying isotopes have different numbers of protons. Instead, stating that proton number is identical and only neutron number differs. Exam tip: Define isotopes using both 'same protons' and 'different neutrons' to secure the mark.

See it

It shows that isotopes of an element share the same proton number and electron arrangement but differ in neutrons and mass number, and that relative atomic mass is a weighted mean.

Key words

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Isotopes

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Relative atomic mass

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See every definition

Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons, so they have the same atomic number but different mass numbers.
Relative atomic mass: The weighted mean of isotope masses, so it need not be a whole number.

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