Explain isotope notation and isotope properties
Identify isotopes from proton and neutron counts and link their equal chemical behaviour and differing physical properties to their structure.
AS Level · Cambridge International Chemistry 9701, 2025–2027 syllabus. Electronic configurations are limited to ground-state atoms and ions of elements H to Kr. Written practice is self-checked; it does not replace cumulative exam practice or human subject-teacher review.
Before you begin
- Use atomic symbols and the Periodic Table, and distinguish an atom from an ion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the defining difference between isotopes of one element?
What you will learn
- Define isotopes using proton and neutron numbers
- Interpret nuclide notation
- Explain chemical similarity and differences in mass and density
Isotopes have the same proton number
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Since proton number defines the element, isotopes share Z but have different nucleon numbers A.
In notation ᴬZX, A is the mass or nucleon number, Z is the atomic or proton number, and X is the element symbol. Neutron number is A − Z. Check that the symbol and Z identify the same element before calculating particles.
Electron structure explains chemical similarity
Neutral atoms of isotopes have the same proton number and therefore the same number and arrangement of electrons. Chemical behaviour depends mainly on electron arrangement, especially the outer electrons, so isotopes of an element have the same chemical properties.
Their different neutron numbers give them different masses. Differences in mass can also produce differences in physical properties such as density. This syllabus comparison is limited to mass and density; do not infer a different chemical reactivity from neutron number alone.
Pause and explain
Why do neutral isotopes of the same element have the same chemical properties?
Worked example
Explain why ³⁵Cl and ³⁷Cl have the same chemical properties but different masses.
Show the worked solution
- Both chlorine isotopes have 17 protons.
- Neutral atoms therefore have the same electron configuration.
- They differ in neutron number: 18 versus 20.
Answer The same proton number gives both neutral atoms the same electron arrangement, so their chemical properties are the same. The ³⁷Cl isotope has two more neutrons and therefore a greater mass.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For ⁶³₂₉Cu and ⁶⁵₂₉Cu, state the proton and neutron numbers and identify which is heavier.
Give me a hint
The lower number gives protons; subtract it from the upper number for neutrons.
Compare my reasoning
- Both have Z = 29.
- Calculate A − Z for each isotope.
- Compare nucleon numbers.
Both have 29 protons. ⁶³Cu has 34 neutrons and ⁶⁵Cu has 36; ⁶⁵Cu is heavier.
Look for these in your work
- Same proton count
- Neutron counts correct
- Greater nucleon number means greater isotope mass
Explain why isotopes of chlorine have the same chemical reactions but can have different densities.
Give me a hint
Link chemical properties to electrons and density to mass.
Compare my reasoning
- Isotopes have equal proton number.
- Neutral atoms therefore have equal electron arrangements.
- Different neutron numbers change mass and may change density.
Their equal proton number gives neutral atoms the same electron arrangement, which determines their chemical properties. Different neutron numbers give different masses, so their physical properties, including density, can differ.
Look for these in your work
- Electron basis for chemistry
- Mass basis for physical difference
A sample contains atoms with 17 protons and either 18 or 20 neutrons. Explain what can and cannot be inferred about their chemistry.
Give me a hint
Identify the element and compare electron arrangements, not neutron numbers.
Compare my reasoning
- Both atom types have proton number 17, so both are chlorine.
- Neutral atoms each have 17 electrons.
- The neutron difference changes mass, not the electron arrangement.
They are chlorine isotopes. Neutral atoms of both types have 17 electrons and the same chemical properties. Their different neutron numbers give different masses; this alone does not justify predicting different chemical reactions.
Look for these in your work
- Same-element identity
- Same neutral electron arrangement
- Inference is limited to syllabus properties
Common mistakes
- Defining isotopes as atoms with different protons
- Reading the upper nuclide number as the proton number
- Claiming isotopes have different chemical properties because their masses differ
What can you explain now?
What is the defining difference between isotopes of one element?
Compare with the explanation
They have different numbers of neutrons
They share proton number but differ in neutron number.
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Make it stick
What stays the same between isotopes, what changes, and how does each fact affect properties?
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