Year 8 Science · Unit 2 · Lesson 08
Challenge Worksheet
Learning Goals
Find the errors
This summary contains exactly three errors. Underline each one and write the corrections below.
A student’s summary
Most carbon atoms have 6 protons and 6 neutrons, but some carbon atoms have an extra proton, which makes them carbon-13. Carbon-14 has 6 protons and 8 neutrons and is radioactive. Carbon-12 is stable and makes up about 99% of all carbon. Because carbon-14 is radioactive, every isotope must be radioactive. Iron-58 (26 protons) and nickel-58 (28 protons) have the same mass number, so they are isotopes of each other.
1. Write the three corrections.
| Isotope | Z | A | Neutrons | Stable or radioactive |
|---|---|---|---|---|
| Iodine-127 | 53 | 127 | 74 | stable |
| Iodine-131 | 53 | 131 | radioactive | |
| Uranium-235 | 92 | 235 | radioactive | |
| Uranium-238 | 92 | 238 | radioactive |
Data: standard isotope data. Iodine-131 is used in medicine; uranium-235 is used as nuclear fuel.
2. Complete the Neutrons column of Table 1. Then, for each pair, calculate how many more neutrons the heavier isotope has, and how much bigger its mass number is.
Extra neutrons: iodine uranium
3. Compare the two differences you found for each pair in Q2. Explain why this result must hold for any two isotopes of the same element, but not for two atoms of different elements.
4. A student looks at the iodine rows and claims: “For every element, the heavier isotope is radioactive and the lighter one is stable.” Use the uranium rows, and the fact that oxygen-18 is stable, to evaluate this claim. What does decide whether an isotope is radioactive?
Someone claims…
“Radioactive isotopes are dangerous, so no isotope should ever be used in medicine. Doctors should find a way to do every scan without them.”
5. Evaluate this claim. Say what is reasonable in it and what is wrong or oversimplified, using examples of isotopes from this lesson. Suggest what information you would need to decide whether a radioactive isotope should be used for a particular scan. Finish with a clear judgement.
Find the errors
1. (i) Carbon-13 has an extra neutron (6 protons, 7 neutrons), not an extra proton; an extra proton would make the atom nitrogen. (ii) Not every isotope is radioactive: many are stable, for example carbon-12. (iii) Iron-58 and nickel-58 are different elements, not isotopes, because they have different numbers of protons. [3 marks: 1 per correction]
Your Turn
2. Neutrons: iodine-131, 78; uranium-235, 143; uranium-238, 146. Extra neutrons and extra mass number: iodine 4 and 4; uranium 3 and 3. [4 marks: 2 neutrons column (1 if one error); 1 neutron differences; 1 mass number differences]
Go Deeper
3. The two differences are equal. A = protons + neutrons, and isotopes of one element have the same protons, so only neutrons can change A: each extra neutron adds exactly 1. Atoms of different elements also differ in protons, so a difference in A could come from protons, neutrons or both. [3 marks: 1 A = p + n; 1 protons fixed for isotopes; 1 why it fails for different elements]
4. The claim fails. Uranium-235 is the lighter uranium isotope, yet it is radioactive (both uranium isotopes are). Oxygen-18 is heavier than oxygen-16, yet it is stable. So being heavier does not make an isotope radioactive. Whether an isotope is radioactive depends on the balance of protons and neutrons in its nucleus. [4 marks: 1 uranium evidence; 1 oxygen-18 evidence; 1 verdict; 1 balance of protons and neutrons]
Show What You Know
5. Marking guide (points expected): 1 reasonable: radiation from radioactive isotopes can harm living cells, so use must be controlled; 1 not all isotopes are radioactive (oxygen-18 is stable and used as a tracer), so “no isotope” goes too far; 1 benefit: technetium-99m (from molybdenum-99, made at ANSTO) helps detect cancer, check heart function and diagnose bone disorders; 1 a ban would lose these diagnoses; 1 information needed, for example the radiation dose, how long the isotope stays in the body, and whether a scan without radiation would work as well; 1 supported judgement, for example: mostly wrong, because radioactive isotopes should be used when the benefit outweighs a small, controlled risk. [6 marks]
Common error to watch for: using “isotope” to mean “radioactive substance”. An isotope is any version of an element with a particular number of neutrons; most everyday atoms are stable isotopes.