Orbitals fill in order of increasing n plus l, and for equal sums in order of increasing n.
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The Madelung rule, also called the n + l rule, states that subshells fill in order of increasing n + l, and when two subshells have the same sum, the one with the lower n fills first. This gives the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s and so on. The rule explains why 4s fills before 3d even though 3d has a lower principal quantum number. It is a guideline rather than an absolute law, and it fails for chromium and copper and for some heavy elements. Its practical use is in writing configurations quickly.
Key ideas: Subshells fill in order of increasing n + l; if n + l ties, the lower n goes first. So 4s (n + l = 4) fills before 3d (n + l = 5), and 4d and 5p tie at 6, so 4d (lower n) fills first. It is easy to apply with the diagonal arrow diagram.
Common mistakes and connections: Students treat the rule as exact; it is an empirical rule with many exceptions and it describes filling order, not orbital energies in ions. Once the 3d is occupied, it drops below 4s. It is applied in Aufbau and explains why the periodic table has d and f blocks after s.
Subshells fill in order of increasing n + l; here 4s (4) fills before 3d (5), and ties go to the lower n. The interactive lesson shows this as a live animation or chart.
The 4s subshell has n + l = 4, which is lower than the 3d subshell's n + l = 5, so 4s fills first.
Orbitals fill in order of increasing n plus l, and for equal sums in order of increasing n.
Madelung Rule is a intermediate topic in Atomic Structure & Periodicity. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Atomic Structure & Periodicity first: Aufbau Principle, Pauli Exclusion Principle, Hund's Rule.
The next topics in Atomic Structure & Periodicity are Electron Configuration and Noble Gas Shorthand.
Yes. The interactive lesson includes a self-check question on Madelung Rule, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Madelung Rule is topic 64 of 115 in Atomic Structure & Periodicity.