Electron Configuration: Subshell Notation

Capacity formula: Each shell n can hold a maximum of 2n² electrons. Shell 1: 2(1)² = 2. Shell 2: 2(2)² = 8. Shell 3: 2(3)² = 18. Shell 4: 2(4)² = 32. This matches the row lengths in the periodic table: 2, 8, 8 (not 18!), 18, 18..., the discrepancy arises because 3d fills after 4s.

Subshells: s (1 orbital, max 2e⁻); p (3 orbitals, max 6e⁻); d (5 orbitals, max 10e⁻). Each orbital holds at most 2 electrons with opposite spins (Pauli exclusion principle). Shell n holds 2n² electrons max: shell 1 → 2, shell 2 → 8, shell 3 → 18.

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The key to electron configuration is knowing the filling order. The energy of subshells does not simply increase with shell number, the 4s subshell is lower in energy than 3d, so it fills first.

The diagonal rule (Madelung's rule) gives a systematic way to remember this: fill in diagonal rows from top-right to bottom-left through a grid of subshells. At HSC level, the key order to know precisely is up to at least 5p.

We just saw the capacity of each subshell type. That raises a question: in what order do electrons fill these subshells, and why does the sequence not always follow increasing shell number? This card answers it → electrons always fill the lowest available energy subshell first (Aufbau); 4s is lower in energy than 3d, so it fills first.

Aufbau filling order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p. 4s fills before 3d because 4s is lower in energy. When writing electron configurations, list subshells in increasing shell number order (not filling order). Exception: in transition-metal ions, 4s electrons are removed before 3d.

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Noble Gas (Abbreviated) Configurations

For elements beyond the first 10, writing the full configuration is tedious. The abbreviation uses the preceding noble gas in brackets, then the remaining electrons:

Anomalous Configurations: Cr and Cu

Chromium (Z=24) and copper (Z=29) have configurations that deviate from the expected pattern. This arises because half-filled and fully-filled d subshells have extra stability:

We just saw the Aufbau filling order. That raises a question: when multiple orbitals of the same energy are available, how do electrons distribute themselves, and are there any exceptions to the standard filling order? This card answers it → Hund's rule governs same-energy orbitals; Cr and Cu are important exceptions due to extra stability of half-filled and fully-filled d subshells.

Three rules for electron configuration: Aufbau fill lowest energy first. Pauli max 2 electrons per orbital, opposite spins (↑↓, never ↑↑). Hund's for degenerate orbitals, fill each singly before pairing (e.g. 2p³ = ↑ ↑ ↑). Noble-gas notation: [Ar]3d⁶4s² for Fe. Anomalies: Cr = [Ar]3d⁵4s¹ and Cu = [Ar]3d¹⁰4s¹ (half-filled/fully-filled d give extra stability, the accepted school explanation, but really a simplification of the very close 3d/4s energies together with exchange-energy and electron-repulsion effects, so treat it as a rule with known exceptions, not a complete theory).

Pause, write the highlighted three rules into your book.

We just saw the three configuration rules and key anomalies. That raises a question: how do you efficiently decode an electron configuration in a multiple-choice question to determine period, group, and block? This card answers it → highest shell number → period; highest shell electron count → group for main-group elements; always verify by summing superscripts to Z.

For multiple choice on electron configurations: superscripts must add to Z (always check). Valence electrons = electrons in the highest-numbered shell for main-group elements. For d-block (e.g. Mn, Z=25 = [Ar]3d⁵4s²): Group = ns + (n−1)d electrons → Group 7. Common trap: 3p holds 6 electrons, never assign more than capacity.

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We just saw how to use configurations to find period and group. That raises a question: how do you write and verify electron configurations in exam answers, especially for exceptions and ions? This card answers it → follow the rules, apply noble-gas notation for efficiency, and handle Cr/Cu exceptions by explaining the extra stability of special d-subshell fillings.

For short-answer configuration questions: write in subshell notation (1s²2s²2p⁶…), use noble-gas shorthand for long configurations, and always verify total = Z. For Cr and Cu exceptions: explain that a half-filled (3d⁵) or fully-filled (3d¹⁰) subshell has extra stability from minimised electron–electron repulsion and maximised exchange energy. For ions: remove 4s electrons first, then 3d.

Pause, copy the highlighted exam strategy into your book before moving on.