The shape of the lesson
By the end of the lesson, learners can
- The definition and the three factors: Every ionisation energy question is answered with the same three ideas.
- Across Period 3: Step along the period in the simulator.
- Down a group: Group 1 runs Li 520, Na 496, K 419, Rb 403 kJ per mole.
- Definition: one mole, gaseous atoms, gaseous 1+ ions, one electron each.
- Across: nuclear charge up, same shell, similar shielding: IE rises.
- Dips: Al (3p above 3s), S (spin-pair repulsion in 3p4).
- Down: further out and more shielded: IE falls.
One picture every idea returns to
First ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions: X(g) → X+(g) + e−. It is larger when the nuclear charge is greater, the outer electron is closer, and shielding is lower. Across Period 3 it generally rises, with dips at aluminium (3p sits above 3s) and sulfur (spin-pair repulsion in 3p4). Down a group it falls because the outer electron is further away and more shielded.
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, The cost of losing an electron.. First ionisation energy measures how tightly an atom holds its outermost electron. 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: The definition and the three factors | Every ionisation energy question is answered with the same three ideas. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Across Period 3 | Step along the period in the simulator. Project tab 6 of the lab (Atomic structure lab, first ionisation energy trend) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Down a group | Group 1 runs Li 520, Na 496, K 419, Rb 403 kJ per mole. | Mix-Pair-Share |
| 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 Down a group, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| Three things lose marks in a definition: forgetting gaseous, forgetting one mole, and writing a 2+ ion for a first ionisation. | In explanations, say shielding or outer electron further from the nucleus; "more electrons" on its own is not an explanation. |
Ionisation energy and its trends: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Define first ionisation energy and write an equation, with state symbols, for the first ionisation energy of chlorine.
Explain why first ionisation energy generally increases across Period 3 from sodium to argon.
The first ionisation energies of magnesium and aluminium are 738 and 578 kJ mol−1. Explain why aluminium's is lower.
The first ionisation energies of phosphorus and sulfur are 1012 and 1000 kJ mol−1. Explain why sulfur's is lower.
Explain why first ionisation energy decreases down Group 2 from beryllium (900 kJ mol−1) to barium (503 kJ mol−1).
Predict which has the higher first ionisation energy, potassium or calcium, and explain your prediction.
Answer key and marking notes
- The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
- Cl(g) → Cl+(g) + e−.
- Nuclear charge increases by one proton for each element across the period.
- The outer electron being removed is in the same shell (n = 3) each time, and shielding by the inner n = 1 and n = 2 shells is approximately constant.
- The outer electron is therefore attracted more strongly and more energy is needed to remove it.
- The electron removed from aluminium is in the 3p sub-shell, whereas magnesium's is in 3s; 3p is higher in energy than 3s.
- The 3p electron is also partly shielded by the 3s electrons, so less energy is needed to remove it despite aluminium's greater nuclear charge.
- Phosphorus (3p3) has one electron in each 3p orbital; in sulfur (3p4) two electrons share one 3p orbital.
- The paired electrons repel each other (spin-pair repulsion), so one of them is removed more easily than an unpaired 3p electron in phosphorus.
- Down the group the outer electron is in a shell further from the nucleus, so the attraction to the nucleus is weaker.
- There are more inner shells, so shielding of the outer electron increases.
- These two effects outweigh the increase in nuclear charge, so less energy is needed to remove the outer electron.
- Calcium has the higher first ionisation energy (590 compared with 419 kJ mol−1).
- Both remove a 4s electron with the same inner shells and similar shielding, but calcium has 20 protons against potassium's 19, so its outer electron is held more strongly.
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 factor does NOT directly affect the first ionisation energy of an atom?
- Athe number of neutrons in the nucleus
- Bthe nuclear charge
- Cthe distance of the outer electron from the nucleus
- Dthe shielding by inner-shell electrons
The bar chart shows the first ionisation energies of eight elements with consecutive proton numbers, labelled with code letters. Which element is in Group 13?
- AQ
- BM
- CT
- DK
The first ionisation energy of helium (2372 kJ mol−1) is much greater than that of hydrogen (1312 kJ mol−1). Which statement explains this?
- AHelium has a nuclear charge of +2 acting on electrons in the same 1s orbital, with almost no shielding
- BHelium's two electrons are paired, and pairing makes an electron harder to remove
- CHelium's electron is further from the nucleus than hydrogen's
- DHelium has a stable full shell, so it has no attraction for its electrons to overcome
Sodium has 11 protons and neon has 10, yet the first ionisation energy of neon is about four times that of sodium. Why?
- ASodium's outer electron is in the 3s sub-shell, further from the nucleus and shielded by two full inner shells
- BSodium has fewer electrons than neon
- CNeon's outer electrons are all paired, which holds them more tightly
- DSodium has a smaller nuclear charge than neon
The first ionisation energy of sodium is 496 kJ mol−1. How much energy is needed to remove one electron from every atom in 2.30 g of gaseous sodium atoms? (Ar: Na = 23.0.)
- A49.6 kJ
- B1141 kJ
- C216 kJ
- D4.96 kJ
The first ionisation energies of zinc and gallium are 906 and 579 kJ mol−1. Zinc is [Ar] 3d10 4s2. Why is gallium's value lower?
- AGallium's outer electron is in a 4p orbital, which is higher in energy than 4s and partly shielded by the 4s electrons
- BGallium has a smaller nuclear charge than zinc
- CGallium's outer electron is one of a spin-paired pair in a 4p orbital
- DGallium's outer electron is removed from the 3d sub-shell
Which shows sodium, magnesium and aluminium in order of increasing first ionisation energy?
- ANa < Al < Mg
- BNa < Mg < Al
- CAl < Na < Mg
- DMg < Al < Na
For which element is the first ionisation energy GREATER than that of the element with the next proton number?
- Anitrogen
- Bcarbon
- Csodium
- Daluminium
Quiz key
Neutrons carry no charge, so changing their number (as in isotopes) leaves the attraction on the outer electron unchanged. Nuclear charge, distance from the nucleus and shielding by inner electrons are the three factors that together decide how strongly the outer electron is held.
L has the highest value, so it is a noble gas, and M, straight after it with the lowest value, is in Group 1. Counting on, N is Group 2 and Q is Group 13; the small drop from N to Q is the 3p-above-3s dip. M is the Group 1 element, T is two places further on in Group 15, and K sits just before the noble gas in Group 17.
Both electrons are removed from 1s at a similar distance, and one 1s electron barely shields the other, so doubling the nuclear charge makes helium's electron much harder to remove. Spin pairing actually adds repulsion and makes removal slightly easier, not harder. Helium's electrons are closer, not further, and saying the shell is ‘stable’ does not name the attraction that must be overcome.
Neon's electron comes from 2p in n = 2, shielded only by 1s2. Sodium's comes from 3s in a new shell, further out and shielded by both n = 1 and n = 2, so it is far easier to remove even though sodium has one more proton. Sodium has more electrons and more protons than neon, not fewer, and electron pairing does not increase the attraction.
Amount = 2.30 / 23.0 = 0.100 mol, so energy = 0.100 × 496 = 49.6 kJ. Multiplying 496 by the mass gives 1141 kJ, dividing 496 by the mass gives 216 kJ, and 4.96 kJ is a factor of ten too small (0.0100 mol).
Gallium is [Ar] 3d10 4s2 4p1. Its single 4p electron is higher in energy than zinc's 4s and is shielded by the 4s pair, so it is removed more easily, exactly as for magnesium and aluminium. Gallium has 31 protons against zinc's 30, a lone 4p electron cannot be spin-paired, and the 3d sub-shell is full and lies below 4p.
Values are Na 496, Al 578 and Mg 738 kJ mol−1. Aluminium falls below magnesium because its outer electron is in the higher-energy 3p sub-shell, but it is still above sodium because of its much greater nuclear charge. Following the general trend blindly gives Na < Mg < Al, and the other two orders put aluminium or magnesium below sodium, whose single 3s electron is the easiest of the three to remove.
Nitrogen (2p3, 1402 kJ mol−1) is above oxygen (2p4, 1314 kJ mol−1) because oxygen's fourth 2p electron is paired and repelled by its partner. Carbon is below nitrogen, sodium below magnesium and aluminium below silicon, all following the rising trend across a period.
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.