Distinguish isotopes from ions and different elements
Compare equal proton numbers and different neutron numbers without relying on electron counts.
Section 5.1.2 · The nucleus
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Use labelled nuclides, supplied count intervals and stated background conditions. Model drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Read nuclide numbers.
- Distinguish nucleus from electrons.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pair is isotopic?
What you will learn
- Compare equal proton numbers and different neutron numbers without relying on electron counts.
- Classify nuclear and electronic changes using the precise isotope definition.
Keep element identity fixed
Isotopes are nuclei of the same element with the same proton number but different neutron numbers, and therefore different nucleon numbers. An element may have more than one isotope. Carbon-12 and carbon-14 both have Z = 6, but their neutron counts are six and eight.
Two nuclei with different Z belong to different elements even if their mass numbers match. Equal mass number alone does not define isotopes. Nor does equal electron count, since ions of different elements can have the same number of electrons.
Separate nuclear and electronic changes
Removing an electron produces an ion without changing which isotope its nucleus is. Changing neutron count while keeping protons fixed describes another isotope; changing protons changes the element. State which inventory changed before naming the category.
Not every isotope is radioactive. The isotope definition describes proton and neutron counts, not stability. Do not infer half-life or decay mode from the word isotope alone. Those properties need evidence for the particular nuclide.
Pause and explain
A carbon-14 atom loses one electron. Has it become carbon-12?
Worked example
Compare ¹²₆C and ¹⁴₆C. Are they isotopes, and how many neutrons does each have?
Show the worked solution
- Both have Z = 6.
- Their A values differ.
- Neutrons are 12 − 6 and 14 − 6.
Answer Isotopes of carbon; 6 and 8 neutrons.
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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.
Explain why carbon-12 and carbon-14 are isotopes.
Give me a hint
Use both the invariant and the changed count.
Compare my reasoning
- Each has six protons.
- They have six and eight neutrons.
- Same element with different neutron counts meets the definition.
Same Z = 6; different neutron counts.
Look for these in your work
- State the same-element condition.
- Name what differs.
Supplied nuclei have (A,Z) = (20,10) and (22,10). Find both neutron counts.
Give me a hint
Apply A − Z to each nucleus.
Compare my reasoning
- First neutrons: 20 − 10 = 10.
- Second neutrons: 22 − 10 = 12.
- Z stays ten, so they are isotopes.
10 and 12 neutrons; isotopes.
Look for these in your work
- Calculate both counts.
- Retain equal Z.
Two supplied species have ten electrons, but Z values 10 and 11. A report calls them isotopes. Correct it.
Give me a hint
Compare nuclear proton counts, not surrounding electrons.
Compare my reasoning
- Their electron counts match.
- Their proton counts differ.
- They are different elements, not isotopes of one element.
Equal electrons do not establish isotopes; different Z means different elements.
Look for these in your work
- Use the nuclear definition.
- Explain why the supplied electron evidence is insufficient.
Common mistakes
- Equal Z is essential.
- Electron loss does not make another isotope.
- Isotope does not mean radioactive.
What can you explain now?
What fixes element identity?
Compare with the explanation
Proton count
Nuclei with the same proton number belong to the same element.
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Make it stick
Classify nuclear and electronic changes using the precise isotope definition.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.