Electronic configuration of all elements

The filling order (Aufbau principle)

Electrons fill subshells from lowest energy up: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. The (n + l) rule gives this order: lower n + l fills first, and for a tie, lower n fills first. That's why 4s (4 + 0 = 4) fills before 3d (3 + 2 = 5).

• Pauli exclusion principle: an orbital holds at most two electrons, with opposite spins. So s holds 2, p holds 6, d holds 10 and f holds 14.
• Hund's rule: in a subshell, electrons occupy orbitals singly before pairing up. Nitrogen's three 2p electrons sit in three separate orbitals.

Exceptions to learn

Half-filled and completely filled subshells are extra stable, so some elements break the order:

Configurations here follow NCERT. For elements beyond 103 they are predicted, not measured. Short forms use the previous noble gas in brackets. See the first 30 with valencies on this page, or explore the periodic table.

Questions students ask

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