Chemistry • Year 11 • Module 1 • Lesson 16
Electron Configuration: Subshell Notation
Lock in the key vocabulary, the four subshell types, the three filling rules, and the Aufbau filling order before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: principal quantum number, subshell, orbital, Aufbau principle, Pauli exclusion principle, Hund’s rule, electron configuration, valence electrons, noble gas core, anomalous configuration. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The quantum number (n) that describes the electron shell or main energy level; higher values mean higher energy and greater distance from the nucleus. | |
| 1.2 | A division within an electron shell defined by the azimuthal quantum number; types are s, p, d, and f, each with a fixed number of orbitals. | |
| 1.3 | A region of space where there is a high probability of finding an electron; each one holds a maximum of two electrons. | |
| 1.4 | “Building up” principle — electrons enter the lowest available energy level first; gives the filling order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… | |
| 1.5 | No two electrons in the same atom can have an identical set of all four quantum numbers; practical result: each orbital holds at most two electrons with opposite spins. | |
| 1.6 | Within a subshell, electrons occupy separate orbitals singly (with parallel spins) before any orbital is doubly occupied; minimises electron repulsion. | |
| 1.7 | The complete description of how electrons are distributed among the subshells of an atom, written as 1s²2s²2p&sup6;… | |
| 1.8 | The electrons in the outermost shell of an atom; they determine chemical reactivity and correspond to the group number for main-group elements. | |
| 1.9 | Abbreviated notation that replaces the inner electrons with the symbol of the preceding noble gas in square brackets, e.g. [Ne]3s²3p³ for phosphorus. | |
| 1.10 | An electron configuration that deviates from the expected Aufbau order; Cr and Cu are the two key HSC examples, due to the extra stability of half-filled and fully-filled d subshells. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 The 4s subshell fills before the 3d subshell because it has a lower energy for most elements. T / F
2.2 A p subshell contains 5 orbitals and can hold a maximum of 10 electrons. T / F
2.3 The Pauli exclusion principle states that an orbital can hold at most 2 electrons, provided they have opposite spins. T / F
2.4 Chromium (Z=24) has the electron configuration [Ar]3d&sup4;4s² according to the Aufbau principle. T / F
2.5 When writing the electron configuration of iron (Z=26), the 3d subshell is written before 4s in the notation, even though 4s filled first. T / F
2.6 An s subshell can hold a maximum of 6 electrons because it has 3 orbitals. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
Aufbau · d · exchange · half-filled · noble gas · parallel · principal · valence
The ___________ quantum number (n) describes which electron shell an electron occupies. According to the ___________ principle, electrons always fill the lowest available energy subshell first. The filling order crosses shell boundaries: the 4s subshell fills before the 3___________ subshell. Electrons in the outermost shell are called ___________ electrons; for main-group elements the number of valence electrons equals the group number. An abbreviated configuration uses the preceding ___________ element in square brackets to represent the inner electrons. Hund’s rule requires that electrons in the same subshell have ___________ spins before pairing occurs. Chromium is anomalous because a ___________ d subshell has extra stability, arising from ___________ energy between electrons of the same spin.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What is the maximum number of electrons that can occupy a d subshell, and why?
4.2 What does Hund’s rule require when writing the orbital diagram for nitrogen (Z=7)?
4.3 Why is the abbreviated electron configuration [Ne]3s²3p&sup5; preferred over the full configuration for chlorine?
4.4 How can you use an element’s electron configuration to determine its group and period in the periodic table?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “governs”, “limits”, “determines”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: Aufbau principle · subshell · orbital · Pauli exclusion principle · Hund’s rule · electron configuration.
6. Complete the subshell summary table
Fill in every empty cell. 6 marks (1 per row)
| Subshell type | Number of orbitals | Max electrons | Example full notation | l value |
|---|---|---|---|---|
| s | 0 | |||
| p | 1 | |||
| d | 2 | |||
| f | 3 |
Q1 — Term–definition match
1.1 principal quantum number • 1.2 subshell • 1.3 orbital • 1.4 Aufbau principle • 1.5 Pauli exclusion principle • 1.6 Hund’s rule • 1.7 electron configuration • 1.8 valence electrons • 1.9 noble gas core • 1.10 anomalous configuration.
Marking: 1 mark per correct match, no half marks.
Q2 — True / false with correction
2.1 True. The 4s subshell has lower energy than 3d for most neutral atoms due to shielding and penetration effects, so it fills first (Aufbau principle).
2.2 False. A p subshell contains 3 orbitals (not 5) and holds a maximum of 6 electrons. It is the d subshell that has 5 orbitals and holds 10 electrons.
2.3 True. The Pauli exclusion principle requires no two electrons in the same atom to share all four quantum numbers; as a result each orbital holds at most two electrons with opposite spins (spin-up ↑ and spin-down ↓).
2.4 False. Chromium (Z=24) has the anomalous configuration [Ar]3d⁵4s¹ (not [Ar]3d⁴4s²), because the extra stability of a half-filled d subshell (all 5 d orbitals singly occupied) outweighs the energy cost of promoting one electron from 4s to 3d.
2.5 True. Although 4s fills before 3d during electron loading, the convention is to write shells in ascending numerical order. Iron’s configuration is written 1s²2s²2p⁶3s²3p⁶3d⁶4s², with 3d before 4s in the written notation.
2.6 False. An s subshell contains only 1 orbital and holds a maximum of 2 electrons (not 6). The 3 orbitals and 6-electron capacity belong to the p subshell.
Q3 — Cloze paragraph
In order: principal / Aufbau / d / valence / noble gas / parallel / half-filled / exchange.
Q4.1 — d subshell capacity
A d subshell contains 5 orbitals; each orbital holds a maximum of 2 electrons (Pauli exclusion principle), giving a maximum of 10 electrons in total across the d subshell.
Q4.2 — Hund’s rule for nitrogen
Nitrogen (Z=7) has the configuration 1s²2s²2p³. Hund’s rule requires that the three 2p electrons each occupy a separate p orbital with parallel spins (↑ ↑ ↑) before any orbital is doubly occupied — not two in one and one in another (↓↑ ↑ empty).
Q4.3 — Abbreviated configuration for Cl
Chlorine (Z=17) has the full configuration 1s²2s²2p⁶3s²3p⁵. The inner 10 electrons (1s²2s²2p⁶) are the same as the noble gas neon, so they can be replaced by [Ne], giving the shorter [Ne]3s²3p⁵. The abbreviated form is quicker to write and immediately highlights the valence electrons (3s²3p⁵ = 7 valence electrons, Group 17).
Q4.4 — Configuration to period and group
The period equals the highest principal quantum number (n) of occupied subshells. The group for main-group elements equals the number of valence electrons (outermost-shell electrons). For transition metals, the group equals the total number of d and s valence electrons (e.g. Fe is [Ar]3d⁶4s² → 8 electrons in 3d+4s → Group 8, Period 4).
Q5 — Sample concept map
Correct maps should include arrows such as:
- Aufbau principle — governs filling order of → subshell
- subshell — contains → orbital
- Pauli exclusion principle — limits capacity of → orbital
- Hund’s rule — determines spin arrangement in → subshell
- Aufbau principle — applied when writing → electron configuration
- electron configuration — is built from → subshell
Award 1 mark per valid labelled arrow (minimum 6, maximum 6 marked).
Q6 — Subshell summary table
| Subshell | Orbitals | Max e² | Example notation | l value |
|---|---|---|---|---|
| s | 1 | 2 | 1s², 2s², 3s² | 0 |
| p | 3 | 6 | 2p⁶, 3p⁴ | 1 |
| d | 5 | 10 | 3d¹⁰, 4d⁵ | 2 |
| f | 7 | 14 | 4f¹⁴ | 3 |
Accept any valid notation examples. 1 mark per complete row.