Electron Configuration and Orbital Filling - Chemistry

1. Fundamental Concepts

Electron Configuration: Describes the distribution of electrons in atomic orbitals, following the Aufbau principle, Pauli exclusion principle, and Hund’s rule.
Atomic Orbitals: Spatial regions where electrons move, categorized into s, p, d, f orbitals with distinct shapes, energies, and electron capacities (s: 2 electrons, p: 6 electrons, d: 10 electrons, f: 14 electrons).
Energy Levels and Sublevels: Energy levels (n=1,2,3…) correspond to electron shells; sublevels (s, p, d, f) are energy subdivisions within a shell, with the energy order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p…

2. Key Concepts

Aufbau Principle: Electrons fill the lowest-energy orbitals first to keep the atom in its most stable state.
Pauli Exclusion Principle: An orbital can hold a maximum of 2 electrons with opposite spins.
Hund’s Rule: In degenerate orbitals (e.g., p³, d⁵), electrons occupy separate orbitals with parallel spins first; fully filled (p⁶, d¹⁰), half-filled (p³, d⁵), and empty orbital states are more stable.
Orbital Filling Order: Follows the energy level crossing rule: 1s→2s→2p→3s→3p→4s→3d→4p→5s→4d→5p→6s…
Shorthand Notation: Use the symbol of a noble gas (in square brackets) to represent the filled inner electron configuration, with only the valence shell configuration written (e.g., Na: [Ne] 3s¹).

3. Examples

Easy

The electron configuration of an oxygen atom (O, atomic number 8) is _______.
- Answer: 1s² 2s² 2p⁴
- Explanation: Oxygen has 8 electrons. Fill orbitals in order: 1s (2 electrons) → 2s (2 electrons) → 2p (remaining 4 electrons).
An s orbital can hold a maximum of _______ electrons, and a p sublevel has _______ degenerate orbitals.
- Answer: 2; 3
- Explanation: The Pauli exclusion principle limits s orbitals to 2 electrons; a p sublevel consists of 3 equivalent orbitals (pₓ, pᵧ, p_z).

Medium

The electron configuration of an iron atom (Fe, atomic number 26) is _______, and its shorthand notation is _______.
- Answer: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶; [Ar] 4s² 3d⁶
- Explanation: Iron has 26 electrons. Fill orbitals up to 3d: 1s²→2s²→2p⁶→3s²→3p⁶→4s²→3d⁶. The noble gas before Fe is Ar (atomic number 18), so shorthand uses [Ar] for inner electrons.
According to Hund’s rule, the 2p orbital electron arrangement of an N atom is _______ (describe with orbital diagram, e.g., ↑↓ ↑ ↑).
- Answer: ↑ ↑ ↑
- Explanation: N has 7 electrons: 1s² 2s² 2p³. Hund’s rule requires electrons in 2p degenerate orbitals to occupy separate orbitals with parallel spins (no pairing until necessary).

Hard

The electron configuration of a chromium atom (Cr, atomic number 24) is _______ (hint: half-filled stability), instead of [Ar] 4s² 3d⁴.
- Answer: [Ar] 4s¹ 3d⁵
- Explanation: Cr follows the half-filled stability rule: one 4s electron moves to 3d, making 3d⁵ (half-filled) and 4s¹, which is more stable than 4s² 3d⁴.
The valence electron configuration of Cu²⁺ (copper ion, atomic number 29) is _______ (note: electron loss order: 4s first, then 3d).
- Answer: 3d⁹
- Explanation: Neutral Cu has configuration [Ar] 4s¹ 3d¹⁰. Cu²⁺ loses 2 electrons: first the 4s¹ electron, then one 3d electron, leaving 3d⁹.

4. Problem-Solving Techniques

Determine Atomic Number: Use the element’s atomic number (= proton number = number of electrons) to confirm the total electrons to fill.

Fill Orbitals in Energy Order: Strictly follow the orbital filling sequence (1s→2s→2p→3s→3p→4s→3d…) to fill electrons one by one.

Apply Special Rules: For special elements (e.g., Cr, Cu), prioritize fully/half-filled stable structures; for ions, cations lose ns electrons first, then (n-1)d electrons.

Verify Total Electrons: After filling, check that the total number of electrons matches the atomic number to avoid omissions or excesses.

Shorthand Skill: Identify the noble gas in the period before the element, use its symbol for inner configuration, and focus on the valence shell (only valence electrons participate in chemical reactions).