The electronic configuration of an element describes how electrons are distributed in atomic orbitals. For the first 30 elements (Hydrogen to Zinc), the configurations follow the Aufbau principle, Pauli exclusion principle, and Hund's rule. Some elements like Cr and Cu have exceptional configurations.
Electronic configurations follow: Aufbau principle, Pauli exclusion principle, Hund's rule.
Order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p...
Chromium (Cr): [Ar] 3d⁵ 4s¹ — half-filled 3d is extra stable.
Copper (Cu): [Ar] 3d¹⁰ 4s¹ — fully-filled 3d is extra stable.
Both Cr and Cu are exceptions to the expected Aufbau filling.
Helium (Z=2): 1s² — first noble gas (complete shell).
Zinc (Z=30): [Ar] 3d¹⁰ 4s² — last element of first d-block series.
Notation: 1s = first shell s orbital; 2s, 2p = second shell; 3s, 3p, 3d = third shell; 4s = fourth shell
| No | Element | Symbol | Configuration |
|---|---|---|---|
| 1 | Hydrogen | H | 1s¹ |
| 2 | Helium | He | 1s² |
| 3 | Lithium | Li | 1s² 2s¹ |
| 4 | Beryllium | Be | 1s² 2s² |
| 5 | Boron | B | 1s² 2s² 2p¹ |
| 6 | Carbon | C | 1s² 2s² 2p² |
| 7 | Nitrogen | N | 1s² 2s² 2p³ |
| 8 | Oxygen | O | 1s² 2s² 2p⁴ |
| 9 | Fluorine | F | 1s² 2s² 2p⁵ |
| 10 | Neon | Ne | 1s² 2s² 2p⁶ |
| 11 | Sodium | Na | [Ne] 3s¹ |
| 12 | Magnesium | Mg | [Ne] 3s² |
| 13 | Aluminium | Al | [Ne] 3s² 3p¹ |
| 14 | Silicon | Si | [Ne] 3s² 3p² |
| 15 | Phosphorus | P | [Ne] 3s² 3p³ |
| 16 | Sulphur | S | [Ne] 3s² 3p⁴ |
| 17 | Chlorine | Cl | [Ne] 3s² 3p⁵ |
| 18 | Argon | Ar | [Ne] 3s² 3p⁶ |
| 19 | Potassium | K | [Ar] 4s¹ |
| 20 | Calcium | Ca | [Ar] 4s² |
| 21 | Scandium | Sc | [Ar] 3d¹ 4s² |
| 22 | Titanium | Ti | [Ar] 3d² 4s² |
| 23 | Vanadium | V | [Ar] 3d³ 4s² |
| 24 | Chromium | Cr | [Ar] 3d⁵ 4s¹ ★ |
| 25 | Manganese | Mn | [Ar] 3d⁵ 4s² |
| 26 | Iron | Fe | [Ar] 3d⁶ 4s² |
| 27 | Cobalt | Co | [Ar] 3d⁷ 4s² |
| 28 | Nickel | Ni | [Ar] 3d⁸ 4s² |
| 29 | Copper | Cu | [Ar] 3d¹⁰ 4s¹ ★ |
| 30 | Zinc | Zn | [Ar] 3d¹⁰ 4s² |
★ Exceptional Configurations:
Chromium (Cr, Z=24): • Expected: [Ar] 3d⁴ 4s² • Actual: [Ar] 3d⁵ 4s¹ • Reason: Half-filled 3d subshell (3d⁵) is extra stable (all spins parallel; symmetric)
Copper (Cu, Z=29): • Expected: [Ar] 3d⁹ 4s² • Actual: [Ar] 3d¹⁰ 4s¹ • Reason: Fully-filled 3d subshell (3d¹⁰) is extra stable
Aufbau Principle: electrons fill orbitals in order of increasing energy Order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d... Memory aid: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p
Pauli Exclusion Principle: each orbital can hold at most 2 electrons with opposite spins
Hund's Rule: electrons fill each orbital singly before pairing (maximum multiplicity) Example: Carbon (2p²) → one electron each in 2px and 2py (not paired in 2px)
Maximum electrons per subshell: • s: 2 electrons (1 orbital) • p: 6 electrons (3 orbitals) • d: 10 electrons (5 orbitals) • f: 14 electrons (7 orbitals)
Chromium (Cr, Z=24): expected [Ar] 3d⁴ 4s², actual [Ar] 3d⁵ 4s¹ — because half-filled 3d⁵ (all spins parallel) gives extra stability. Copper (Cu, Z=29): expected [Ar] 3d⁹ 4s², actual [Ar] 3d¹⁰ 4s¹ — because fully-filled 3d¹⁰ gives extra stability. In both cases, one electron from 4s moves to 3d to achieve a more stable configuration.
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