The shape of the lesson
By the end of the lesson, learners can
- Same protons, different neutrons: Pick an element in the simulator and add or remove neutrons.
- Why isotopes react the same: Extended.
- Relative atomic mass: Extended.
- Isotopes: same proton number, different mass number.
- Same chemistry: same number of electrons and the same configuration.
- Ar: sum of (mass number × % abundance) ÷ 100.
- Sense check: Ar lies nearer the most abundant isotope.
One picture every idea returns to
Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons, so they have different mass numbers. (Extended) Isotopes of an element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration. The relative atomic mass, Ar, is the average mass of the isotopes weighted by their percentage abundances.
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 mass.. Not every atom of an element is identical. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Same protons, different neutrons | Pick an element in the simulator and add or remove neutrons. Project tab 2 of the lab (Atomic structure lab, isotopes) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Why isotopes react the same | Extended. | Pairs |
| 21 to 30 min | Teach and check: Relative atomic mass | Extended. | Rally Coach |
| 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, 13 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 Relative atomic mass, 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.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
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 |
|---|---|
| Do not just average the mass numbers. | For chlorine, (35 + 37) ÷ 2 = 36 is wrong because there are three times as many 35 Cl atoms as 37 Cl atoms. Each mass must be weighted by its abundance, and the total divided by 100, not by 2. |
Isotopes: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Define the term isotopes.
State the number of protons, neutrons and electrons in an atom of 126C and in an atom of 146C, and explain why they are isotopes.
(Extended) Explain why 35Cl and 37Cl react in the same way with sodium.
(Extended) Chlorine is 75% 35Cl and 25% 37Cl. Calculate the relative atomic mass of chlorine.
(Extended) Boron is 20% 10B and 80% 11B. Calculate the relative atomic mass of boron.
(Extended) Bromine is 50.5% 79Br and 49.5% 81Br. Calculate the relative atomic mass of bromine to one decimal place.
Answer key and marking notes
- Different atoms of the same element that have the same number of protons,
- but different numbers of neutrons.
- Carbon-12: 6 protons, 6 neutrons, 6 electrons.
- Carbon-14: 6 protons, 8 neutrons, 6 electrons.
- Same number of protons (same element) but different numbers of neutrons, so they are isotopes.
- Both have 17 electrons with the same configuration, 2,8,7, so the same number of outer-shell electrons.
- Chemical reactions depend on the electrons, so both gain one electron to form Cl− in the same way; the extra neutrons do not affect this.
- (35 × 75) + (37 × 25) = 2625 + 925 = 3550.
- Ar = 3550 ÷ 100 = 35.5.
- (10 × 20) + (11 × 80) = 200 + 880 = 1080.
- Ar = 1080 ÷ 100 = 10.8.
- (79 × 50.5) + (81 × 49.5) = 3989.5 + 4009.5 = 7999.
- Ar = 7999 ÷ 100 = 79.99 = 80.0.
8 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.
Hydrogen has three isotopes: hydrogen-1, hydrogen-2 and hydrogen-3. Which statement about them is correct?
- AEach has one proton and one electron
- BEach has one neutron
- CHydrogen-3 has three protons
- DThey are different elements because their masses are different
Uranium has proton number 92. How many neutrons are in an atom of uranium-235?
- A143
- B92
- C235
- D327
Four atoms are described: W (8 protons, 8 neutrons), X (8 protons, 10 neutrons), Y (9 protons, 9 neutrons), Z (10 protons, 8 neutrons). Which two atoms are isotopes of the same element?
- AW and X
- BW and Z
- CX and Y
- DY and Z
A sample of gallium contains 60% gallium-69 and 40% gallium-71. What is the relative atomic mass of gallium in this sample?
- A69.8
- B70.0
- C70.2
- D69.0
A sample of rubidium contains only rubidium-85 and rubidium-87, and its relative atomic mass is 85.5. What percentage of the rubidium atoms in the sample are rubidium-87?
- A25%
- B75%
- C50%
- D2.5%
Heavy water, D2O, is water in which the hydrogen-1 atoms are replaced by hydrogen-2 (deuterium) atoms. Which property of heavy water is most likely to be different from that of ordinary water?
- Aits density
- Bthe number of electrons in each molecule
- Cthe number and type of covalent bonds in each molecule
- Dwhether it reacts with sodium
The relative atomic mass of chlorine is 35.5. A student concludes that some chlorine atoms have a mass number of 35.5. Why is the student wrong?
- AMass numbers are always whole numbers; 35.5 is a weighted average of the isotopes chlorine-35 and chlorine-37
- BThe extra 0.5 is the mass of the electrons, which is not included in the mass number
- CEach chlorine atom contains half a neutron, so 35.5 is its proton number
- D35.5 is the mass of a chlorine molecule divided by two
Neon is 90% neon-20 and 10% neon-22 (ignore the small amount of neon-21). What is the relative atomic mass of neon?
- A20.2
- B21.0
- C21.8
- D20.0
Quiz key
All hydrogen atoms have proton number 1, so each isotope has one proton and, as a neutral atom, one electron. The mass numbers 1, 2 and 3 come from having 0, 1 and 2 neutrons, so they do not all have one neutron. Hydrogen-3 has one proton and two neutrons. Different masses do not make different elements; the proton number decides the element.
Neutrons = mass number − proton number = 235 − 92 = 143. 92 is the number of protons (and electrons), 235 is the total number of protons and neutrons, and 327 comes from adding instead of subtracting.
W and X both have 8 protons, so both are oxygen, but they have different numbers of neutrons (8 and 10): they are isotopes. W and Z have the same number of neutrons but different numbers of protons, so they are different elements. X and Y both have mass number 18 but different proton numbers (8 and 9). Y and Z have 9 and 10 protons, so they are also different elements.
Ar = (69 × 60 + 71 × 40) ÷ 100 = (4140 + 2840) ÷ 100 = 6980 ÷ 100 = 69.8. The answer is closer to 69 because gallium-69 is more common. 70.0 is the simple average that ignores abundance; 70.2 comes from swapping the two percentages; 69.0 is just the mass of the more common isotope.
(85(100 − x) + 87x) ÷ 100 = 85.5, so 8500 + 2x = 8550, giving 2x = 50 and x = 25%. Check: (85 × 75 + 87 × 25) ÷ 100 = (6375 + 2175) ÷ 100 = 85.5. 75% is the abundance of rubidium-85, not rubidium-87; a value of 85.5 lies nearer 85 than 87, so rubidium-85 must be the more common isotope. 50% would give 86.0, and 2.5% is a decimal-place slip.
Deuterium has an extra neutron, so each D2O molecule is heavier than H2O and heavy water is denser (a physical property that depends on mass). Both isotopes of hydrogen have one electron, so each molecule still has 10 electrons, forms the same two covalent bonds to oxygen, and reacts in the same way with sodium. Chemical properties depend on electrons, which are the same for all isotopes of an element.
A mass number counts whole protons and neutrons, so it must be a whole number. Chlorine is a mixture of isotopes (mainly chlorine-35, with some chlorine-37), and 35.5 is the average mass weighted by their abundances. Seventeen electrons have a relative mass of only about 0.01, not 0.5; there is no such thing as half a neutron; and chlorine's proton number is 17. Halving the mass of a Cl2 molecule just gives the average atom again, which does not explain why it is not a whole number.
Ar = (20 × 90 + 22 × 10) ÷ 100 = (1800 + 220) ÷ 100 = 20.2. It is close to 20 because neon-20 is much more common. 21.0 is the unweighted average, 21.8 comes from swapping the abundances, and 20.0 ignores neon-22 altogether.
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.