Chemistry • Year 11 • Module 1 • Lesson 16
Electron Configuration: Subshell Notation
Apply the three filling rules to real elements, interpret electron configuration data, spot errors in student work, and connect configurations to periodic table position.
1. Interpret electron configuration data — first-row transition metals
The table below gives partial information for eight first-row transition metals (Z = 21–29, excluding Cu). Complete every empty cell. 10 marks (1 per completed row)
| Element (symbol) | Z | Full electron configuration | Abbreviated configuration | Number of 3d electrons |
|---|---|---|---|---|
| Scandium (Sc) | 21 | [Ar]3d¹4s² | ||
| Titanium (Ti) | 22 | 2 | ||
| Vanadium (V) | 23 | 1s²2s²2p⁶3s²3p⁶3d³4s² | ||
| Chromium (Cr) | 24 | 5 | ||
| Manganese (Mn) | 25 | [Ar]3d⁵4s² | ||
| Iron (Fe) | 26 | 6 | ||
| Cobalt (Co) | 27 | [Ar]3d⁷4s² | ||
| Nickel (Ni) | 28 | 1s²2s²2p⁶3s²3p⁶3d⁸4s² | ||
| Copper (Cu) | 29 | 10 (anomalous) |
1.1 Explain why chromium (Z=24) and copper (Z=29) deviate from the expected Aufbau filling pattern. What special stability is involved? 2 marks
2. Sequence the filling order
The subshells below are listed in alphabetical order, not energy order. Rewrite them in the correct Aufbau filling sequence, numbered 1 (lowest energy) to 13 (highest). 6 marks (1 per correct 2 positions)
Subshells to order: 2p · 2s · 3d · 3p · 3s · 4d · 4f · 4p · 4s · 5p · 5s · 1s · 6s
| Position | Subshell | Position | Subshell | Position | Subshell | Position | Subshell |
|---|---|---|---|---|---|---|---|
| 1 | 4 | 7 | 10 | ||||
| 2 | 5 | 8 | 11 | ||||
| 3 | 6 | 9 | 12 | ||||
| 13 | |||||||
3. Spot the error — student configurations
Each student configuration below contains one or more errors. For each, identify every error and write the correct configuration. Explain which rule is violated. 9 marks (3 per sub-question: 1 error identified, 1 correct config, 1 rule)
3.1 Student claim: Calcium (Z=20): 1s²2s²2p⁶3s²3p⁶3d²
Error(s):
Correct configuration:
Rule violated:
3.2 Student claim: Phosphorus (Z=15): 1s²2s²2p⁶3s²3p³ — with the three 3p electrons shown as (↑↓ ↑ empty) in the orbital diagram.
Error(s):
Correct orbital arrangement:
Rule violated:
3.3 Student claim: Chlorine (Z=17): 1s²2s²2p⁵3s²3p⁶
Error(s):
Correct configuration:
Rule violated:
4. Predict and justify — fireworks and electron configuration
Sydney’s New Year’s Eve fireworks display uses strontium salts for red bursts and barium salts for green bursts. In both cases, electrons in the metal atoms are promoted to excited states when the compound burns, then fall back to lower energy levels, releasing photons of specific visible wavelengths.
5 marks
4.1 Strontium has Z=38. Write the abbreviated electron configuration for ground-state strontium. Identify the subshell that loses an electron first when Sr forms a 2+ ion (Sr²♠). 2 marks
4.2 Barium has Z=56. Write the abbreviated electron configuration for ground-state barium. Using the pattern from strontium, predict the abbreviated configuration of Ba²♠. 2 marks
4.3 Both Sr and Ba are in Group 2. What feature of their electron configurations explains why they both readily lose exactly two electrons to form 2+ ions? 1 mark
5. Compare: full configuration vs abbreviated configuration
Complete the two-column table for five features. 10 marks (1 per cell)
| Feature | Full subshell configuration | Abbreviated (noble gas core) configuration |
|---|---|---|
| Example for iron (Z=26) | ||
| What it shows | ||
| Advantage | ||
| Limitation | ||
| When it is required in HSC |
Q1 — First-row transition metals table
Sc (21): Full: 1s²2s²2p⁶3s²3p⁶3d¹4s² • 3d electrons: 1. Ti (22): Full: 1s²2s²2p⁶3s²3p⁶3d²4s² • Abbreviated: [Ar]3d²4s². V (23): Abbreviated: [Ar]3d³4s² • 3d electrons: 3. Cr (24): Full: 1s²2s²2p⁶3s²3p⁶3d⁵4s¹ • Abbreviated: [Ar]3d⁵4s¹ (anomalous). Mn (25): Full: 1s²2s²2p⁶3s²3p⁶3d⁵4s² • 3d electrons: 5. Fe (26): Full: 1s²2s²2p⁶3s²3p⁶3d⁶4s² • Abbreviated: [Ar]3d⁶4s². Co (27): Full: 1s²2s²2p⁶3s²3p⁶3d⁷4s² • 3d electrons: 7. Ni (28): Abbreviated: [Ar]3d⁸4s² • 3d electrons: 8. Cu (29): Full: 1s²2s²2p⁶3s²3p⁶3d¹⁰4s¹ • Abbreviated: [Ar]3d¹⁰4s¹ (anomalous).
Marking: 1 mark per complete, correct row.
Q1.1 — Anomalous configurations
Chromium (Z=24): a half-filled 3d subshell ([Ar]3d⁵4s¹) has extra stability due to exchange energy — all five d orbitals are singly occupied with parallel spins, maximising same-spin interactions and minimising repulsion. One electron is promoted from 4s to 3d to achieve this. Copper (Z=29): a fully-filled 3d subshell ([Ar]3d¹⁰4s¹) has similarly enhanced stability for the same reasons. Award 1 mark for each anomaly explained with reference to half-filled or fully-filled d subshell stability.
Q2 — Filling order sequence
1 = 1s • 2 = 2s • 3 = 2p • 4 = 3s • 5 = 3p • 6 = 4s • 7 = 3d • 8 = 4p • 9 = 5s • 10 = 4d • 11 = 5p • 12 = 6s • 13 = 4f
Marking: Award 1 mark per correct pair of positions (1–2, 3–4, 5–6, 7–8, 9–10, 11–12; position 13 is a bonus). Accept small deviations due to 4f/5d crossover at heavier elements.
Q3.1 — Calcium error
Error: The student placed the last 2 electrons in 3d instead of 4s. The 4s subshell has lower energy than 3d and fills first (Aufbau principle). The total electron count of 20 is correct, but the last two electrons belong in 4s, not 3d. Correct configuration: 1s²2s²2p⁶3s²3p⁶4s² (abbreviated: [Ar]4s²). Rule violated: Aufbau principle — lowest available energy subshell fills first; 4s is lower in energy than 3d.
Q3.2 — Phosphorus orbital error
Error: The orbital diagram shows one 3p orbital doubly occupied (↑↓) while another 3p orbital is empty. This violates Hund’s rule. Correct orbital arrangement: All three 3p orbitals each contain one electron with parallel spins: (↑ ↑ ↑) across px, py, pz. The subshell notation 3p³ is correct; the orbital diagram must show three singly occupied orbitals. Rule violated: Hund’s rule — electrons occupy separate orbitals singly with parallel spins before any orbital is doubly occupied.
Q3.3 — Chlorine error
Error: The 2p subshell has only 5 electrons (2p⁵) when it should be full (2p⁶); the 3p subshell has 6 electrons (3p⁶) when it should have only 5 (3p⁵). Total electron count = 17 is correct, but the Aufbau principle requires 2p to be completely filled (6 electrons) before any electrons enter 3p. Correct configuration: 1s²2s²2p⁶3s²3p⁵ (abbreviated: [Ne]3s²3p⁵). Rule violated: Aufbau principle — lower-energy subshells (2p) must be fully filled before higher ones (3p) are occupied.
Q4.1 — Strontium configuration
Strontium (Z=38): [Kr]5s² (abbreviated). Sr²♠: the two 5s electrons are removed first, giving [Kr] (abbreviated) or 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶ (full). 1 mark for correct ground-state config; 1 mark for correctly identifying the 5s subshell as the source of the lost electrons.
Q4.2 — Barium configuration
Barium (Z=56): [Xe]6s² (abbreviated). Ba²♠: removing the two 6s electrons gives [Xe]. 1 mark each.
Q4.3 — Group 2 commonality
Both Sr and Ba have exactly two electrons in their outermost s subshell (5s² and 6s² respectively). These two outer electrons are relatively easily removed because they are in the highest energy level and are shielded from the nucleus by inner electrons. Losing two electrons gives the stable noble gas core configuration, which is the basis for their consistent 2+ ion formation. (1 mark for identifying two outermost s electrons / ns² valence configuration.)
Q5 — Full vs abbreviated configuration comparison
Iron example: Full: 1s²2s²2p⁶3s²3p⁶3d⁶4s² • Abbreviated: [Ar]3d⁶4s².
What it shows: Full = all subshells from 1s outward • Abbreviated = only the valence electrons and d electrons beyond the noble gas core.
Advantage: Full = nothing is hidden; complete electron accounting • Abbreviated = quicker to write; emphasises valence/reactive electrons.
Limitation: Full = tedious for heavy elements; easy to make arithmetic errors • Abbreviated = inner electrons not shown; requires knowing which noble gas precedes the element.
When required in HSC: Full = usually requested for elements Z≤36 in questions that test Aufbau knowledge • Abbreviated = widely accepted for any element; preferred for transition metals.
Marking: 1 mark per correctly completed cell (10 total, 2 per row).