Isotopes
CIE A-Level PhysicsΒ· 9702 Learning Outcome 11.1.2Β· 10 min read
1. Definition and Key Properties of Isotopesβ βββββ± 3 min
Isotopes
Isotopes are nuclides of the same chemical element, meaning they have the same number of protons in the nucleus, but different numbers of neutrons.
Example:
Carbon has two common isotopes: carbon-12 (6 protons, 6 neutrons) and carbon-14 (6 protons, 8 neutrons).
Chemical reactions involve interactions between an atom's electrons. For neutral atoms, electron number equals proton number, so isotopes have identical chemical properties. They can only be separated by physical methods that exploit their different masses, not chemical reactions.
Neutral chlorine has two common isotopes: chlorine-35 and chlorine-37. Compare the number of protons, neutrons, and electrons in each.
- 1
Chlorine is element 17, so proton number for all chlorine isotopes. Proton number defines the element, so proton count is identical:
- 2
Nucleon (mass) number , so rearrange to get neutrons =
- 3
Calculate for chlorine-35 ():
- 4
Calculate for chlorine-37 ():
- 5
For neutral atoms, electron number equals proton number:
2. Nuclide Notation for Isotopesβ β ββββ± 3 min
CIE exams always use standard nuclide notation to represent isotopes, so you must be able to interpret and write this notation correctly.
Standard Nuclide Notation
The standard format for isotopes: = total nucleon number (protons + neutrons), = proton number, = chemical symbol of the element.
Write the nuclide notation for an oxygen isotope with 10 neutrons.
- 1
Oxygen always has a proton number , so that is our bottom number.
- 2
Calculate nucleon number: , this is our top number.
- 3
Oxygen's chemical symbol is O, so the full notation is:
Test your understanding of notation:
What do and represent?
Different elements with the same number of neutrons
Isotopes of the same element
Different elements with different mass numbers
The same nuclide written differently
Reveal answer
1 βSame element symbol X means same proton number Z, different top numbers mean different mass, so these are isotopes.
3. Common Uses of Isotopesβ β ββββ± 4 min
Isotopes have many practical applications that are often used as context for exam questions. Key examples you should recognize include: carbon-14 for radiocarbon dating, cobalt-60 for cancer radiotherapy, and uranium-235 for nuclear power fuel.
Explain why plants absorb both carbon-12 and carbon-14 from the atmosphere during photosynthesis.
- 1
Photosynthesis is a chemical reaction that depends on the chemical properties of carbon.
- 2
Chemical properties are determined by electron arrangement, which depends on proton number. Both carbon isotopes have the same number of protons, so identical chemical properties.
- 3
Places cannot distinguish between the two isotopes, so both are absorbed and incorporated into plant tissue during photosynthesis.
4. Common Pitfalls
Wrong move:
Mixing up A and Z, counting the bottom number as neutrons instead of protons
Why:
Students often forget which number goes where in standard nuclide notation
Correct move:
Remember: the larger number (nucleon count) is always at the top, smaller number (proton count) at the bottom. Neutrons = top number minus bottom number.
Wrong move:
Claiming isotopes of the same element have different chemical properties
Why:
Students confuse differences in physical properties with chemical properties
Correct move:
Chemical properties depend on electron arrangement, which matches proton number, so isotopes have identical chemical properties. Only physical properties differ.
Wrong move:
Assuming electron number always equals proton number for isotopes
Why:
Questions often ask about ions, not just neutral atoms
Correct move:
For an ion with charge , electrons = . For charge , electrons = .
Wrong move:
Thinking different isotopes occupy different positions in the periodic table
Why:
The periodic table is ordered by mass number in many student memory models
Correct move:
The periodic table is ordered by proton number, so all isotopes of an element share the same position.
5. Quick Reference Cheatsheet
Property | For isotopes of the same element |
|---|---|
Proton number (Z) | Same |
Number of protons | Same |
Number of neutrons | Different |
Nucleon number (A) | Different |
Electrons (neutral atom) | Same |
Chemical properties | Identical |
Mass / Density | Different |
Periodic table position | Same |
6. Frequently Asked
Do isotopes have the same chemical properties?
Yes. Chemical properties depend on electron arrangement, which matches proton number, so all isotopes of an element have identical chemical properties.
Why are some isotopes radioactive?
Radioactivity comes from an unstable neutron-to-proton ratio in the nucleus. Isotopes with an unbalanced ratio decay radioactively, while isotopes with a balanced ratio are stable.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 12
Identify isotope from nuclear notation
- 2023 Β· 22
Calculate neutrons in carbon isotope
Going deeper
What's Next
Understanding isotopes is the foundational concept for all further topics in nuclear physics, including radioactive decay, nuclear reactions, and mass defect and binding energy. You will also apply this knowledge when solving half-life problems, interpreting nuclear reaction equations, and even calculating relative atomic mass in chemistry. This sub-topic almost always appears as an easy 1-2 mark question in your AS exam, so mastering it guarantees you those easy marks. Next, explore the core topics below to build out your nuclear physics knowledge.