Quantum Chemistry
Electron Configuration
How electrons fill orbitals — the address of every electron in an atom
Electron configuration is the distribution of electrons across atomic orbitals, written as a sequence like 1s² 2s² 2p⁶. Determined by three rules: (1) Aufbau — fill lowest-energy orbitals first. (2) Pauli — max 2 electrons per orbital, opposite spins. (3) Hund — half-fill degenerate orbitals before pairing. Configuration determines all chemistry: bonding, ionization, magnetism, color. Noble gas core notation (e.g., [Ne] 3s²) shortens long configurations. Anomalies: Cr is [Ar] 3d⁵ 4s¹ (half-filled stability), Cu is [Ar] 3d¹⁰ 4s¹ (full d).
- Aufbau order1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s
- Pauli exclusionMax 2 electrons per orbital, opposite spins
- Hund's ruleFill degenerate orbitals singly first
- Notationnl^x where n=shell, l=subshell, x=electrons
- Hydrogen1s¹
- Carbon1s² 2s² 2p²
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Why configuration matters
- Bonding. Predicts which atoms form which bonds.
- Periodic trends. Explains patterns in atomic properties.
- Reactivity. Stable configurations resist change.
- Spectroscopy. Transitions between configurations give spectra.
- Magnetism. Unpaired electrons make atoms paramagnetic.
- Catalysis. d-orbital configurations enable catalysis.
- Materials. Conductivity, color depend on configuration.
Common misconceptions
- Fill by shell number. 4s fills before 3d due to energy.
- All atoms follow Aufbau exactly. Anomalies (Cr, Cu, etc.).
- Hund's rule is optional. Lowest-energy state requires it.
- Ions remove from same orbital as added. Cations lose s before d.
- Valence = outer shell only for main group. Transition metals include d.
- Pauli is just a counting rule. Fundamental QM principle.
Frequently asked questions
How do you write an electron configuration?
Fill orbitals in Aufbau order until all electrons assigned. For sulfur (Z=16): 1s² 2s² 2p⁶ 3s² 3p⁴. Total: 2+2+6+2+4 = 16. Or use noble gas core: [Ne] 3s² 3p⁴ where [Ne] = 1s² 2s² 2p⁶. Shortcut: locate element on periodic table; the block (s, p, d, f) tells you the highest filled orbital.
What are the rules for filling?
Three rules. Aufbau: lowest energy first. Pauli exclusion: max 2 electrons/orbital. Hund's rule: in degenerate orbitals (same energy, e.g., three 2p), one electron each before pairing. The energy order isn't strictly by shell — 4s fills before 3d because 4s is lower energy in neutral atoms.
What's the noble gas core?
Shortcut for writing configurations. Replace inner electrons with the previous noble gas symbol in brackets. Iron (Z=26): full = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Short = [Ar] 3d⁶ 4s². [Ar] = first 18 electrons. Saves writing for heavy atoms.
Why is chromium 3d⁵ 4s¹?
Anomaly — predicted [Ar] 3d⁴ 4s², actual [Ar] 3d⁵ 4s¹. Reason: half-filled d-subshell has extra stability (exchange energy). Promoting one electron from 4s to 3d gives all five d-orbitals one electron each. Similar anomaly: copper [Ar] 3d¹⁰ 4s¹ (full d-subshell stable).
What's a valence electron?
Electron in the outermost shell. For sodium [Ne] 3s¹: one valence electron (3s¹). For chlorine [Ne] 3s² 3p⁵: seven valence electrons. Valence electrons participate in bonding. Group number on periodic table corresponds to valence electron count for main-group elements.
Why does configuration matter?
Determines all chemistry. (1) Bonding: valence electrons. (2) Reactivity: stability of full/half-full subshells. (3) Magnetism: unpaired electrons → paramagnetic. (4) Color: d-d transitions in transition metals. (5) Periodic trends: ionization energy reflects how tightly held the outer electrons are.
How do ions differ?
Cations: lose electrons from highest occupied orbital first. Na (1s² 2s² 2p⁶ 3s¹) → Na⁺ (1s² 2s² 2p⁶). Anions: add electrons to next available. Cl (3s² 3p⁵) → Cl⁻ (3s² 3p⁶). Transition metal cations: lose s before d. Fe ([Ar] 3d⁶ 4s²) → Fe²⁺ ([Ar] 3d⁶), not [Ar] 3d⁴ 4s².