The shape of the lesson
By the end of the lesson, learners can
- Shells, sub-shells and orbitals: Rotate the 3D orbitals in the simulator.
- Writing a configuration: Use the configuration builder to fill electrons one at a time.
- Configurations of ions: Cations lose electrons from the highest energy occupied sub-shell; anions add electrons to the lowest available one.
- Order: 1s 2s 2p 3s 3p 4s 3d 4p. Capacity 2, 6, 10.
- Hund: singly first, then pair.
- Ions: cations lose 4s before 3d; anions fill the next empty orbital.
- Exceptions: Cr 3d54s1, Cu 3d104s1.
One picture every idea returns to
Shell n holds s, p, d and f sub-shells up to n of them; an s sub-shell has one orbital, p has three, d has five, and each orbital holds two electrons with opposite spins. Sub-shells fill in order of increasing energy, 1s 2s 2p 3s 3p 4s 3d 4p, so 4s fills before 3d. Within a sub-shell, electrons occupy orbitals singly before pairing. Chromium and copper break the pattern to give half-filled and full 3d sub-shells.
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, Where the electrons live.. Shells split into sub-shells, sub-shells into orbitals, and every orbital holds two electrons. 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: Shells, sub-shells and orbitals | Rotate the 3D orbitals in the simulator. Project tab 3 of the lab (Atomic structure lab, orbital shapes) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Writing a configuration | Use the configuration builder to fill electrons one at a time. Project tab 4 of the lab (Atomic structure lab, electron configuration builder) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Configurations of ions | Cations lose electrons from the highest energy occupied sub-shell; anions add electrons to the lowest available one. | Think-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, 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 Configurations of ions, 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.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
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 |
|---|---|
| 4s is filled before 3d and emptied before 3d. | Writing Fe 2+ as [Ar] 3d 4 4s 2 is the classic error. And chromium is 3d 5 4s 1, not 3d 4 4s 2; copper is 3d 10 4s 1, not 3d 9 4s 2. |
Shells, orbitals and electron configuration: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Write the full electron configurations of phosphorus (Z = 15), calcium (Z = 20) and nickel (Z = 28).
Describe how the 2p electrons are arranged in the orbitals of a nitrogen atom and of an oxygen atom, and state the rule you have used.
Write the electron configuration of copper, and explain why it is not [Ar] 3d9 4s2.
Write the electron configurations of the ions Cu2+, Br− and Ti3+.
Explain why the 4s sub-shell is filled before the 3d sub-shell in potassium and calcium, yet the 4s electrons are the first to be removed when iron forms Fe2+.
An element has the electron configuration [Ar] 3d10 4s2 4p3. Identify the element and state its group in the Periodic Table.
Answer key and marking notes
- P: 1s22s22p63s23p3.
- Ca: 1s22s22p63s23p64s2.
- Ni: 1s22s22p63s23p63d84s2.
- Nitrogen, 2p3: one electron in each of the three 2p orbitals, all with parallel spins, so three unpaired electrons.
- Oxygen, 2p4: the fourth electron pairs up in one of the 2p orbitals, leaving two unpaired electrons.
- Hund's rule: electrons occupy orbitals of equal energy singly, with parallel spins, before any orbital is doubly occupied.
- Cu: [Ar] 3d10 4s1.
- A completely filled 3d sub-shell gives extra stability, so one 4s electron moves into 3d; the 4s and 3d sub-shells are very close in energy, which makes this possible.
- Cu2+: copper is [Ar] 3d10 4s1; remove the 4s electron then one 3d electron, giving [Ar] 3d9.
- Br−: bromine (35) is [Ar] 3d10 4s2 4p5; add one electron to 4p, giving [Ar] 3d10 4s2 4p6.
- Ti3+: titanium (22) is [Ar] 3d2 4s2; remove both 4s electrons then one 3d, giving [Ar] 3d1.
- In potassium and calcium the empty 4s sub-shell is slightly lower in energy than 3d, so 4s fills first.
- Once 3d contains electrons its energy drops below that of 4s, so in iron the 4s electrons are the highest-energy electrons and are removed first.
- Total electrons = 18 + 10 + 2 + 3 = 33, so Z = 33: arsenic.
- The outer configuration is 4s2 4p3, five outer electrons, so it is in Group 15.
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.
How many orbitals are there in total in the third principal quantum shell (n = 3)?
- A9
- B3
- C18
- D8
In which order do the sub-shells fill in a potassium or calcium atom, from lowest to highest energy?
- A1s 2s 2p 3s 3p 4s
- B1s 2s 2p 3s 3p 3d
- C1s 2s 3s 2p 3p 4s
- D1s 2p 2s 3p 3s 4s
What is the electron configuration of the Zn2+ ion? (Zn: Z = 30.)
- A[Ar] 3d10
- B[Ar] 3d8 4s2
- C[Ar] 3d9 4s1
- D[Ar] 3d10 4s2
The orbital box diagram shows the ground-state electron arrangement of a species. Which species is it?
- ACo2+
- BCo
- CFe2+
- DNi2+
Which atom has four unpaired electrons in its ground state?
- AFe
- BC
- CMn
- DNi
Which species has the same electron configuration as a V3+ ion? (Z: Ca = 20, Ti = 22, V = 23, Cr = 24.)
- ATi2+
- BCa
- CCr3+
- DTi
Which ion has no unpaired electrons in its ground state? (Z: Cr = 24, Fe = 26, Cu = 29.)
- ACu+
- BCu2+
- CFe3+
- DCr3+
How many electrons occupy s orbitals in a ground-state selenium atom (Z = 34)?
- A8
- B6
- C2
- D4
Quiz key
n = 3 contains 3s (1 orbital), 3p (3 orbitals) and 3d (5 orbitals): 1 + 3 + 5 = 9. The value 3 counts sub-shells rather than orbitals, 18 is the maximum number of electrons the shell can hold (two per orbital), and 8 is the number of electrons in the third shell of argon, which leaves 3d empty.
Potassium (19) and calcium (20) put their outer electrons into 4s after 3p, because the empty 4s is lower in energy than 3d. Placing 3d before 4s would give a d-block configuration to a Group 1 or 2 element. Within a shell s always fills before p, so any order with 2p or 3p ahead of the matching s is wrong.
Zinc is [Ar] 3d10 4s2. On ionisation the 4s electrons are lost first, leaving [Ar] 3d10. Taking the electrons from 3d gives [Ar] 3d8 4s2, splitting them gives [Ar] 3d9 4s1, and [Ar] 3d10 4s2 is the neutral atom.
The diagram shows [Ar] 3d7: 18 + 7 = 25 electrons with 4s empty, so it is a 2+ ion of cobalt (Z = 27, [Ar] 3d7 4s2). A cobalt atom would still have two electrons in 4s. Fe2+ is [Ar] 3d6 and Ni2+ is [Ar] 3d8, one electron fewer and one more than shown.
Iron is [Ar] 3d6 4s2: five 3d electrons go in singly and the sixth pairs up, leaving four unpaired. Carbon has four outer electrons but only two unpaired (2p2, since 2s2 is paired). Manganese (3d5) has five unpaired, and nickel (3d8) has two.
V is [Ar] 3d3 4s2, so V3+ loses both 4s electrons and one 3d: [Ar] 3d2. Ti2+ is also [Ar] 3d2. A calcium atom has the same 20 electrons but arranges them as [Ar] 4s2. Cr3+ is [Ar] 3d3, and a titanium atom is [Ar] 3d2 4s2.
Copper is [Ar] 3d10 4s1, so Cu+ is [Ar] 3d10 with every 3d orbital full and paired. Cu2+ is [Ar] 3d9 with one unpaired electron, Fe3+ is [Ar] 3d5 with five, and Cr3+ is [Ar] 3d3 with three.
Se is 1s22s22p63s23p63d104s24p4. The s sub-shells 1s, 2s, 3s and 4s each hold 2, giving 8. The value 6 is the number of outer-shell electrons (4s24p4), 2 counts only 4s, and 4 is the number of 4p electrons.
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.