Electrons fill orbitals in order of increasing energy, starting from the lowest available.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
The Aufbau principle, from the German for building up, states that electrons occupy the lowest-energy orbitals available before filling higher ones. The order of filling follows the Madelung rule, which orders subshells by increasing n + l. The principle predicts the ground-state electron configurations of most elements correctly. Exceptions such as chromium and copper occur because subshells of similar energy differ only slightly. It provides the logical basis for writing electron configurations.
Key ideas: Fill electrons into the lowest energy orbitals first. Order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. Steps: find the electron count (Z minus charge), then fill subshell by subshell to capacity. Example: Fe (26) is 1s2 2s2 2p6 3s2 3p6 4s2 3d6.
Common mistakes and connections: Students remove electrons from 3d before 4s when making cations; actually 4s electrons leave first, so Fe2+ is 3d6. Another mistake is thinking Aufbau has no exceptions, but it fails for Cr, Cu, and others. It works with Pauli and Hund to produce full configurations.
Electrons fill the lowest levels first; note 4s sits below 3d so it fills before the 3d subshell in potassium and calcium. The interactive lesson shows this as a live animation or chart.
The filling order begins 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, so the 4s orbital fills before the 3d.
Electrons fill orbitals in order of increasing energy, starting from the lowest available.
Aufbau Principle is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Atomic Structure & Periodicity first: Shapes of d and f Orbitals, Radial and Angular Nodes, Electron Shells and Subshells.
The next topics in Atomic Structure & Periodicity are Pauli Exclusion Principle and Hund's Rule.
Yes. The interactive lesson includes a self-check question on Aufbau Principle, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Aufbau Principle is topic 61 of 115 in Atomic Structure & Periodicity.