The shape of the lesson
By the end of the lesson, learners can
- Why the energy keeps rising: Choose an element in the simulator and remove its electrons one at a time.
- Reading the jumps: Aluminium's first three ionisation energies are 578, 1817 and 2745 kJ per mole.
- Electron affinity: Ionisation energy removes an electron; electron affinity adds one.
- Always increasing: more positive ion, same nuclear charge, fewer electrons.
- Jump: next electron comes from an inner shell, closer and less shielded.
- Group: electrons removed before the first jump = outer electrons.
- Electron affinity: first usually exothermic, second always endothermic.
One picture every idea returns to
Successive ionisation energies always increase: each electron leaves an ion that is more positive, so the same nuclear charge holds fewer electrons more tightly and the ion shrinks. A large jump appears when the next electron comes from an inner shell, closer to the nucleus and less shielded. The number of electrons removed before the first large jump equals the number of outer-shell electrons, which is the group number. Electron affinity, X(g) + e− → X−(g), is exothermic for the first electron but endothermic for a second, because a negative ion repels the incoming electron.
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, Read the jumps.. Strip the electrons from one atom one at a time and the energy needed climbs steadily, then leaps every time you break into a new shell. 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: Why the energy keeps rising | Choose an element in the simulator and remove its electrons one at a time. Project tab 5 of the lab (Atomic structure lab, successive ionisation energies) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Reading the jumps | Aluminium's first three ionisation energies are 578, 1817 and 2745 kJ per mole. | Pairs |
| 21 to 30 min | Teach and check: Electron affinity | Ionisation energy removes an electron; electron affinity adds one. | Jigsaw |
| 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, 15 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 Electron affinity, 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.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
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 |
|---|---|
| Count the electrons removed before the jump, not the position of the jump itself. | A large increase between the 2nd and 3rd ionisation energies means two outer electrons, Group 2. And successive ionisation energies are always positive and always increase; a smaller number than the one before is a sign you have misread the table. |
Successive ionisation energies and electron affinity: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why the successive ionisation energies of an element always increase.
The first four ionisation energies of magnesium are 738, 1451, 7733 and 10 541 kJ mol−1. Explain the pattern in terms of the electronic structure of magnesium.
An element has successive ionisation energies of 1086, 2353, 4621, 6223, 37 831 and 47 277 kJ mol−1. Deduce its group and identify the element, given that it is in Period 2.
Describe the shape of a graph of log10(ionisation energy) against the number of electrons removed for silicon (Z = 14), and explain why a logarithmic scale is used.
Define first electron affinity and write an equation, with state symbols, for the first electron affinity of fluorine.
The first electron affinity of chlorine is −349 kJ mol−1. The second electron affinity of oxygen is +798 kJ mol−1. Explain the difference in sign.
Answer key and marking notes
- Each electron is removed from an ion that is more positive than the last, so the remaining electrons are attracted more strongly by the same nuclear charge.
- With fewer electrons there is less repulsion between them, the ion becomes smaller, and the electrons are closer to the nucleus.
- Magnesium is 1s22s22p63s2. The first two electrons are removed from the 3s sub-shell, so the increase from 738 to 1451 is modest.
- The third electron is removed from the n = 2 shell, which is much closer to the nucleus and shielded only by the 1s electrons, so there is a large jump to 7733.
- The fourth electron is also from n = 2, so the rise from 7733 to 10 541 is again modest.
- The large jump is between the 4th and 5th values, so the element has four outer electrons and is in Group 14.
- A Period 2 element with four outer electrons (2s22p2) is carbon.
- Four points rising gently (the 3s and 3p electrons), then a large jump to eight points rising gently (the n = 2 shell), then a second large jump to two points (the 1s electrons).
- The jumps occur where the next electron comes from a shell closer to the nucleus with less shielding.
- The values range from hundreds to hundreds of thousands of kJ per mole; a log scale compresses this range so that both the small steps within a shell and the large jumps between shells can be seen on one graph.
- The enthalpy change when one mole of gaseous atoms each gain one electron to form one mole of gaseous 1− ions.
- F(g) + e− → F−(g).
- In the first electron affinity the electron is added to a neutral atom and is attracted by the nuclear charge, so energy is released and the change is exothermic.
- In the second electron affinity the electron is added to an ion that is already negative, O−.
- The incoming electron is repelled by the negative charge, so energy must be supplied to overcome the repulsion and the change is endothermic.
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.
The first and second ionisation energies of magnesium are 738 and 1451 kJ mol−1. What is the energy needed to convert one mole of Mg(g) into one mole of Mg2+(g)?
- A2189 kJ
- B1451 kJ
- C713 kJ
- D738 kJ
The graph shows log10 of all the successive ionisation energies of a Period 3 element. Which element is it?
- Amagnesium
- Bberyllium
- Csodium
- Daluminium
An element X has the first five ionisation energies 801, 2427, 3660, 25 026 and 32 827 kJ mol−1. What is the most likely formula of its chloride?
- AXCl3
- BXCl
- CXCl2
- DXCl4
Which equation represents the second electron affinity of sulfur?
- AS−(g) + e− → S2−(g)
- BS(g) + 2e− → S2−(g)
- CS−(aq) + e− → S2−(aq)
- DS2−(g) → S−(g) + e−
Potassium (Z = 19) has 19 successive ionisation energies. Between which two is the FIRST large jump?
- Abetween the 1st and 2nd
- Bbetween the 2nd and 3rd
- Cbetween the 7th and 8th
- Dbetween the 9th and 10th
The second ionisation energy of sodium (4562 kJ mol−1) is much greater than that of magnesium (1451 kJ mol−1), even though magnesium has more protons. Why?
- ASodium's second electron is removed from the n = 2 shell, whereas magnesium's second comes from 3s
- BSodium has a smaller nuclear charge, so its electrons are held more tightly
- CThe Na+ ion has a spin-paired 3s electron that is hard to remove
- DMagnesium's second electron is removed from a 3p orbital
Which element has the greatest SECOND ionisation energy?
- Alithium
- Bberyllium
- Cboron
- Dcarbon
The first electron affinity of fluorine (−328 kJ mol−1) is less exothermic than that of chlorine (−349 kJ mol−1). Which is the best explanation?
- AFluorine's n = 2 shell is very compact, so the incoming electron is strongly repelled by the electrons already there
- BFluorine has a smaller nuclear charge, so it attracts the incoming electron less
- CChlorine has more shielding, and shielding increases the attraction for an incoming electron
- DFluorine is more electronegative, so it attracts electrons less strongly
Quiz key
Mg(g) → Mg+(g) needs 738 kJ and Mg+(g) → Mg2+(g) needs 1451 kJ, so the total is 738 + 1451 = 2189 kJ. The second ionisation energy alone (1451 kJ) already starts from Mg+, 713 kJ is the difference rather than the sum, and 738 kJ only reaches Mg+.
The points fall into groups of 2, 8 and 2: two outer electrons in n = 3, eight in n = 2 and two in n = 1, giving 12 electrons, which is magnesium. Beryllium is also in Group 2 but has only four electrons (2, 2). Sodium would show one electron before the first jump, and aluminium three.
The large jump is between the 3rd and 4th values (3660 to 25 026), so X has three outer electrons and is in Group 13 (it is boron), forming BCl3. XCl and XCl2 would suit an element with a jump after the 1st or 2nd value, and XCl4 would need four electrons before the jump.
The second electron affinity adds one electron to each gaseous 1− ion: S−(g) + e− → S2−(g). Adding two electrons at once combines the first and second steps, aqueous ions are not used in the definition, and removing an electron from S2− is the reverse process.
Potassium is 1s22s22p63s23p64s1. Only one electron is in n = 4, so the 2nd electron already comes from n = 3 and the first big jump is between the 1st and 2nd values. The jump between the 9th and 10th exists too (n = 3 to n = 2), but it is the second one. A jump after the 2nd would suit Group 2, and after the 7th a Group 17 element.
Na+ is 1s22s22p6, so its next electron comes from n = 2, much closer to the nucleus and shielded only by 1s. Mg+ is still 1s22s22p63s1, so its second electron is the other 3s electron. A smaller nuclear charge would lower an ionisation energy, not raise it; Na+ has no 3s electrons left; and magnesium has no 3p electrons.
Li+ is 1s2, so lithium's second electron comes from the innermost shell with no shielding: about 7298 kJ mol−1. Be+, B+ and C+ still have n = 2 electrons to lose (about 1757, 2427 and 2353 kJ mol−1). Choosing carbon because it has the largest nuclear charge ignores which shell the electron comes from.
In the tiny fluorine atom the added electron enters an n = 2 shell already crowded with seven electrons, and the extra electron-electron repulsion outweighs its closeness to the nucleus. The greater nuclear charge of chlorine is largely cancelled by its extra inner shell, so that option does not explain the anomaly. Shielding reduces attraction rather than increasing it, and fluorine's high electronegativity means it attracts bonding electrons strongly, not weakly.
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.