The shape of the lesson
By the end of the lesson, learners can
- What an isotope is: Set the abundance of each isotope of chlorine in the simulator and watch the relative atomic mass move.
- Why the chemistry is identical: Chemical reactions involve the electrons: losing them, gaining them, sharing them.
- Notation, and the decimals in the Periodic Table: Write the nucleon number top left and the proton number bottom left.
- Definition: same number of protons, different number of neutrons.
- Chemistry: identical, because the electron configuration is identical.
- Ar: sum of (mass × fraction) over all isotopes.
- Reverse: let the unknown fraction be x and solve one equation.
One picture every idea returns to
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so the same proton number Z and a different nucleon number A. They have the same electron configuration and therefore the same chemical properties. Physical properties that depend on mass, such as density and rate of diffusion, differ slightly. Relative atomic mass is the weighted mean of the isotopic masses, which is why it is rarely a whole number.
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.. Two atoms can share a proton number and still weigh differently. 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: What an isotope is | Set the abundance of each isotope of chlorine in the simulator and watch the relative atomic mass move. Project tab 2 of the lab (Atomic structure lab, isotopes and relative atomic mass) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Why the chemistry is identical | Chemical reactions involve the electrons: losing them, gaining them, sharing them. | Pairs |
| 21 to 30 min | Teach and check: Notation, and the decimals in the Periodic Table | Write the nucleon number top left and the proton number bottom left. | Cooperative Concept Mapping |
| 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, 14 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 Notation, and the decimals in the Periodic Table, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 and electrons; only the neutron number differs. | Saying that isotopes have a different atomic number, or a different number of electrons, loses the mark. And the mass number is a property of one atom; the relative atomic mass is a property of the natural mixture. |
Isotopes: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Define the term isotopes, and state why the relative atomic mass of most elements is not a whole number.
Copper exists as 63Cu and 65Cu. Describe two ways in which atoms of these isotopes are the same and one way in which they differ.
Explain why isotopes of an element have the same chemical properties but slightly different physical properties.
Magnesium consists of 24Mg (79.0%), 25Mg (10.0%) and 26Mg (11.0%). Calculate its relative atomic mass to two decimal places.
Bromine has two isotopes, 79Br and 81Br, and a relative atomic mass of 79.9. Calculate the percentage abundance of each isotope.
An ion contains 20 protons, 20 neutrons and 18 electrons. Write its full nuclide symbol including the charge.
Answer key and marking notes
- Isotopes are atoms of the same element (same proton number) with different numbers of neutrons (different nucleon numbers).
- Relative atomic mass is a weighted average of the masses of the naturally occurring isotopes, so it is usually not a whole number.
- Both have 29 protons (the same proton number).
- Both have 29 electrons in the same configuration, so they have the same chemical properties.
- They differ in neutron number: 34 neutrons in 63Cu and 36 in 65Cu, so 65Cu has the greater mass.
- Chemical properties depend on the electron configuration, which is the same for all isotopes of an element because they have the same number of protons and electrons.
- Physical properties such as density and rate of diffusion depend on mass, and isotopes have different masses because they contain different numbers of neutrons.
- Ar = (24 × 79.0 + 25 × 10.0 + 26 × 11.0) / 100 = (1896 + 250 + 286) / 100.
- Ar = 2432 / 100 = 24.32.
- Let the fraction of 81Br be x, so the fraction of 79Br is (1 − x).
- 79(1 − x) + 81x = 79.9, so 79 + 2x = 79.9 and x = 0.45.
- Abundances: 45% 81Br and 55% 79Br.
- 20 protons is calcium; A = 20 + 20 = 40; charge = 20 − 18 = 2+.
- Symbol: 4020Ca2+.
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.
Which is the correct definition of the relative atomic mass, Ar, of an element?
- AThe weighted mean mass of an atom of the element compared with 1/12 of the mass of an atom of carbon-12
- BThe mass of the most abundant isotope compared with 1/12 of the mass of an atom of carbon-12
- CThe total number of protons and neutrons in one atom of the element
- DThe mean mass of an atom of the element compared with the mass of one hydrogen atom
Hydrogen has three isotopes: 1H, 2H and 3H. Which statement is correct?
- AAll three react with oxygen to form water of formula H2O, but with different molar masses
- BAn atom of 3H contains three electrons
- C2H has a different electron configuration from 1H
- DThe three isotopes have different proton numbers
Gallium consists of 60.1% 69Ga and 39.9% 71Ga. What is the relative atomic mass of gallium, to three significant figures?
- A69.8
- B70.0
- C70.2
- D69.0
The mass spectrum of a sample of silicon is shown. What is the relative atomic mass of silicon, to two decimal places?
- A28.11
- B29.00
- C28.13
- D28.00
Heavy water is D2O, where D is 2H. Which quantity is the same for one molecule of D2O and one molecule of ordinary H2O?
- Athe number of electrons
- Bthe relative molecular mass
- Cthe number of neutrons
- Dthe number of nucleons
Natural neon is 90.5% 20Ne and 0.3% 21Ne, and the rest is 22Ne. What is the relative atomic mass of neon, to two decimal places?
- A20.19
- B21.00
- C18.16
- D20.10
Chlorine is 75% 35Cl and 25% 37Cl. In the mass spectrum of Cl2, molecular ion peaks appear at m/z 70, 72 and 74. What is the ratio of their heights?
- A9 : 6 : 1
- B3 : 2 : 1
- C9 : 3 : 1
- D3 : 1 : 1
Tellurium (proton number 52) has Ar = 127.6, but iodine (proton number 53) has Ar = 126.9. Which statement explains why the element with fewer protons has the greater relative atomic mass?
- AThe naturally occurring isotopes of tellurium have, on average, more neutrons per atom than iodine
- BTellurium atoms contain more electrons than iodine atoms
- CTellurium really has more protons than iodine, which the Periodic Table order hides
- DIodine has only one isotope, so its Ar must be a whole number
Quiz key
Ar is a weighted mean over all the naturally occurring isotopes, measured on the carbon-12 scale (1/12 of a 12C atom). Using only the most abundant isotope ignores the others, the proton plus neutron count is the nucleon number of a single atom, and the hydrogen atom is not the modern reference standard.
All three isotopes have 1 proton and 1 electron (1s1), so they react in the same way and form water of the same formula; only the mass differs because the neutron number is 0, 1 or 2. The 3 in 3H is the nucleon number, not the electron count, and the electron configuration and proton number are identical for all three.
Ar = (69 × 60.1 + 71 × 39.9) / 100 = (4146.9 + 2832.9) / 100 = 69.8. The value 70.0 is a simple unweighted average, 70.2 comes from attaching the 60.1% to the heavier isotope, and 69.0 uses only the more abundant isotope.
Ar = (28 × 92.2 + 29 × 4.7 + 30 × 3.1) / 100 = (2581.6 + 136.3 + 93.0) / 100 = 28.11. The value 29.00 is the unweighted mean of 28, 29 and 30, 28.13 comes from swapping the abundances of the two small peaks, and 28.00 ignores the minor isotopes entirely.
Both molecules have 1 + 1 + 8 = 10 electrons, because isotopes have identical electron counts. Each D atom carries one extra neutron, so D2O has 10 neutrons against 8, 20 nucleons against 18, and a relative molecular mass of about 20 rather than 18.
22Ne makes up 100 − 90.5 − 0.3 = 9.2%. Ar = (20 × 90.5 + 21 × 0.3 + 22 × 9.2) / 100 = (1810 + 6.3 + 202.4) / 100 = 20.19. The value 21.00 is the unweighted mean, 18.16 leaves 22Ne out of the total, and 20.10 attaches the 9.2% to 21Ne and the 0.3% to 22Ne.
The probabilities are 3/4 for 35Cl and 1/4 for 37Cl. m/z 70: (3/4)2 = 9/16; m/z 72: 2 × (3/4)(1/4) = 6/16, the factor 2 because 35-37 and 37-35 both count; m/z 74: (1/4)2 = 1/16. Leaving out the factor 2 gives 9 : 3 : 1, and 3 : 2 : 1 or 3 : 1 : 1 treat the peaks as if they followed the atomic ratio directly.
Mass comes almost entirely from protons plus neutrons. Tellurium's abundant isotopes (such as 128Te and 130Te) carry many neutrons, so its weighted mean exceeds that of iodine despite having one fewer proton. A neutral tellurium atom has 52 electrons, fewer than iodine's 53, and the Periodic Table is ordered by proton number, so tellurium does have fewer protons. Iodine is essentially a single isotope, but its Ar of 126.9 shows the value is still not a whole number.
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.