The shape of the lesson
By the end of the lesson, learners can
- Add a neutron: Keep the protons fixed and change the neutrons to move between isotopes of the same element.
- What stays, what changes: The proton number is fixed, so the element is the same; only the neutrons differ.
- Definition: same proton number, different nucleon number.
- Identify: matching proton numbers means same element, so isotopes.
- Chemistry: the same, because it depends on the electrons.
One picture every idea returns to
Isotopes are atoms of the same element, so they have the same proton number, but they have different numbers of neutrons and so different nucleon numbers. Isotopes of an element are chemically identical.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Same element, different weight.. Carbon is carbon whether its nucleus carries six neutrons or eight. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Add a neutron | Keep the protons fixed and change the neutrons to move between isotopes of the same element. Project tab 3 of the lab (Isotopes) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: What stays, what changes | The proton number is fixed, so the element is the same; only the neutrons differ. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 12 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in What stays, what changes, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Isotopes have the same number of protons, which is why they are the same element. | It is the number of neutrons, not the protons, that differs between isotopes. |
Isotopes: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Define the word isotope.
Carbon-12 and carbon-14 are isotopes of carbon (proton number 6). State the number of neutrons in each.
Three atoms have these proton and nucleon numbers: atom J (proton 8, nucleon 16), atom K (proton 8, nucleon 18), atom L (proton 7, nucleon 15). State which two are isotopes of each other and explain how you can tell.
Explain why two isotopes of the same element react in exactly the same way chemically.
State one physical property that is different between two isotopes of an element, and explain the cause.
A student says that adding a neutron to a carbon atom turns it into a different element. Explain why this is wrong.
Answer key and marking notes
- Isotopes are atoms of the same element (same proton number). ✓
- They have different numbers of neutrons (different nucleon number). ✓
- Carbon-12: 12 - 6 = 6 neutrons. ✓
- Carbon-14: 14 - 6 = 8 neutrons. ✓
- J and K are isotopes of each other. ✓
- They have the same proton number (8), so they are the same element, but different nucleon numbers. ✓
- Chemical behaviour depends on the electrons. ✓
- Isotopes have the same number of protons, so the same number of electrons, so the same chemistry. ✓
- Their mass is different. ✓
- This is because one isotope has more neutrons, giving it a larger nucleon number. ✓
- The element is decided by the proton number, not the neutron number. ✓
- Adding a neutron keeps the proton number at 6, so it is still carbon, just a heavier isotope. ✓
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What are isotopes of an element?
- Aatoms with the same number of protons but different numbers of neutrons
- Batoms with the same number of neutrons but different numbers of protons
- Catoms with different numbers of electrons and protons
- Datoms of completely different elements
Two isotopes of carbon are carbon-12 and carbon-14. How do their chemical properties compare?
- AThey have the same chemical properties, because they have the same number of electrons.
- BThey have completely different chemical properties.
- CCarbon-14 is not really carbon.
- DThey behave like two different elements.
Quiz key
Isotopes have the same proton number (so they are the same element) but different numbers of neutrons, giving different nucleon numbers. Changing the proton number would make it a different element.
Isotopes have the same number of protons and therefore the same number of electrons, so their chemical properties are the same. They differ only in mass, because of the extra neutrons.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.