AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Atoms of the same element can have different numbers of neutrons; these…
Full specification wording
Mass number, atomic number and isotopes: Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
Every atom of an element has the same number of protons, so the atomic number is fixed.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 5 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Every atom of an element has the same number of protons, so the atomic number is fixed. The neutron number can vary, giving atoms of the same element with different mass numbers; these are isotopes. For example, chlorine has isotopes with mass numbers 35 and 37: each has 17 protons, but one has 18 neutrons and the other 20 neutrons. Because isotopes of an element have identical proton counts, they have identical electron counts in neutral atoms and therefore the same chemical properties; their physical properties and relative masses differ because the neutron number changes the mass. The relative atomic mass quoted for an element is a weighted mean of its isotopes' masses, so it is often not a whole number.
What earns the marks
- State that isotopes are atoms of the same element, so they have the same number of protons and the same atomic number.
- State that isotopes differ in the number of neutrons, so they have different mass numbers.
- Calculate the neutron number of an isotope by subtracting the atomic number from the mass number.
- Explain that neutral isotopes of an element have the same electron arrangement, so their chemical properties are the same.
- Explain that different neutron numbers give different masses, so physical properties such as density or rate of diffusion can differ.
- Interpret or calculate a weighted mean relative atomic mass from given isotope abundances.
Common mistakes and how to put them right
Saying isotopes have different numbers of protons.
Put it right by stating that the proton number is fixed for an element and only the neutron number changes.
Confusing mass number with relative atomic mass.
Put it right by describing mass number as a count of protons plus neutrons in one atom, while relative atomic mass is a weighted mean for the element.
Writing an isotope as a charge, such as ³⁵Cl⁻.
Put it right by using mass number and atomic number notation, for example ³⁵₁₇Cl, and reserving charge symbols for ions.
Exam tips
- When asked to define isotopes, include both the same element or same proton number and the different neutron number in one sentence.
- For calculations, write the proton, neutron and electron counts separately before combining them, so an arithmetic slip is easy to spot.
- If asked why isotopes react similarly, link the answer to the electron arrangement rather than to the nucleus.
Try the lesson
The start of the Mass number, atomic number and isotopes: 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.
Have a go first
A sample of chlorine contains atoms that all have 17 protons, but some have 18 neutrons and others have 20 neutrons. What name is given to these different atoms of chlorine?
Listen
Every atom of an element has the same number of protons, so the atomic number is fixed. The neutron number can vary, giving atoms of the same element with different mass numbers; these are isotopes. For example, chlorine has isotopes with mass numbers 35 and 37: each has 17 protons, but one has 18 neutrons and the other 20 neutrons. Because isotopes of an element have identical proton counts, they have identical electron counts in neutral atoms and therefore the same chemical properties; their physical properties and relative masses differ because the neutron number changes the mass. The relative atomic mass quoted for an element is a weighted mean of its isotopes' masses, so it is often not a whole number. A strong answer earns marks for these points. State that isotopes are atoms of the same element, so they have the same number of protons and the same atomic number. State that isotopes differ in the number of neutrons, so they have different mass numbers. Calculate the neutron number of an isotope by subtracting the atomic number from the mass number. Explain that neutral isotopes of an element have the same electron arrangement, so their chemical properties are the same. A common slip: Saying isotopes have different numbers of protons. Instead, stating that the proton number is fixed for an element and only the neutron number changes. Exam tip: When asked to define isotopes, include both the same element or same proton number and the different neutron number in one sentence.
See it
Key words
Say what each word means before you turn the card.
Isotopes
Try answering aloud or in your head before turning the card.
Atomic number
Try answering aloud or in your head before turning the card.
Mass 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
- Isotopes
- Atoms of the same element that have different numbers of neutrons.
- Atomic number
- The number of protons, which is fixed for every atom of an element.
- Mass number
- A count of protons plus neutrons in one atom, which differs between isotopes.
- Relative atomic mass
- A weighted mean of an element's isotopes' masses, so it is often not a whole number.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Mass number, atomic number and isotopes: Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge