The Aufbau principle states that in a ground-state atom, electrons fill subshells in order of increasing energy (1s, 2s, 2p, 3s...), which is how you write ground-state electron configurations for atoms and ions on the AP Chemistry exam (Topic 1.5).
"Aufbau" is German for "building up," and that's exactly what the principle describes. You build an atom's electron configuration by placing electrons into the lowest-energy subshell available, then the next lowest, and so on until you've placed every electron. That filling order (1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p...) is why oxygen's ground-state configuration is 1s²2s²2p⁴ and not some scrambled arrangement.
The energy ordering isn't arbitrary. It comes from Coulomb's law (EK 1.5.A.2). Electrons closer to the positively charged nucleus feel a stronger attraction, so lower shells are lower in energy. The Aufbau principle is basically Coulomb's law turned into a filling recipe. It also gives you the shell/subshell picture from EK 1.5.A.3, where inner electrons are core electrons and outer electrons are valence electrons. One catch the AP exam loves: the principle has famous exceptions. Chromium is [Ar]4s¹3d⁵ and copper is [Ar]4s¹3d¹⁰, because half-filled and fully filled d subshells are unusually stable. And when transition metals form ions, the 4s electrons leave before the 3d electrons, even though 4s filled first.
This term lives in Unit 1: Atomic Structure and Properties, Topic 1.5, and it's named directly in learning objective 1.5.A: represent the ground-state electron configuration of an atom or its ions using the Aufbau principle. That makes it one of the few principles the CED requires you to apply by name. Electron configurations are also the foundation for almost everything that follows in Unit 1. Photoelectron spectroscopy, periodic trends, and valence electron reasoning all assume you can write a correct ground-state configuration first. If your Aufbau filling is wrong, every downstream answer about ionization energy or atomic radius wobbles with it.
Keep studying AP Chemistry Unit 1
Electron Configuration (Unit 1)
The Aufbau principle is the rule; the electron configuration is the result. Every configuration you write, like [Ne]3s²3p⁵ for chlorine, is the Aufbau principle in action. The shorthand noble-gas notation just hides the core electrons that filled first.
Coulomb's Law (Unit 1)
Aufbau isn't a memorized rule from nowhere. Subshells closer to the nucleus are lower in energy because attraction between opposite charges gets stronger as distance shrinks. When an MCQ asks WHY electrons fill 1s before 2s, Coulomb's law is the answer.
Ground State vs. Excited State (Unit 1)
Aufbau only describes the ground state, the lowest-energy arrangement. An excited state (like 1s²2s¹2p¹ for beryllium) breaks the filling order because an electron absorbed energy and jumped up. Spotting which configuration is excited is a classic question format.
Core Electrons (Unit 1)
Because Aufbau fills inner shells first, it automatically sorts electrons into core (inner, filled early) and valence (outer, filled last). That core/valence split is what drives reactivity and the periodic trends you'll use for the rest of Unit 1.
You'll mostly see the Aufbau principle in multiple-choice questions and short FRQ parts that ask you to write or evaluate ground-state configurations. Common question shapes include determining the configuration for an atom given its atomic number, counting unpaired electrons in a ground-state atom, identifying which configuration violates the normal filling order, and working backward from an ion to its neutral atom. That last one is sneaky. If a transition metal ion is [Ar]3d⁶ as a 2+ ion, the neutral atom is [Ar]4s²3d⁶ (iron), because the 4s electrons are removed first. Also know the chromium and copper exceptions cold. Cr is [Ar]4s¹3d⁵, not [Ar]4s²3d⁴, and questions explicitly flag or test this. No released FRQ has required the phrase "Aufbau principle" verbatim, but configuration skills show up constantly in PES and periodicity questions, so the principle is doing quiet work all over Unit 1.
Both govern electron configurations, but they answer different questions. The Aufbau principle tells you WHICH subshell fills next (lowest energy first). Hund's rule tells you HOW electrons arrange WITHIN a subshell (one electron per orbital, all with parallel spins, before any pairing). When you count unpaired electrons, you need both: Aufbau gets you to the right subshell, Hund's rule spreads the electrons out inside it. A question about nitrogen having three unpaired 2p electrons is testing Hund's rule, not Aufbau.
The Aufbau principle says ground-state electrons fill subshells from lowest energy to highest, following the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.
The filling order is explained by Coulomb's law, since electrons closer to the nucleus feel a stronger attraction and sit at lower energy.
Chromium ([Ar]4s¹3d⁵) and copper ([Ar]4s¹3d¹⁰) are the must-know exceptions, because half-filled and fully filled d subshells are extra stable.
When transition metals form cations, electrons leave the 4s subshell before the 3d subshell, even though 4s filled first.
Any configuration that skips a lower-energy subshell, like 1s²2s¹2p¹, describes an excited state, not a ground state.
Learning objective 1.5.A names the Aufbau principle directly, so you're expected to use it to write configurations for both atoms and ions.
It's the rule that electrons in a ground-state atom fill the lowest-energy subshells first, in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on. It's named in AP Chem learning objective 1.5.A as the tool for writing ground-state electron configurations.
No. Chromium and copper are the famous exceptions you need for the exam. Chromium is [Ar]4s¹3d⁵ instead of [Ar]4s²3d⁴, and copper is [Ar]4s¹3d¹⁰, because half-filled and completely filled d subshells are unusually stable.
Aufbau picks which subshell fills next (lowest energy first); Hund's rule says electrons spread out into separate orbitals within that subshell before pairing up. Nitrogen's three unpaired 2p electrons come from Hund's rule, while the fact that 2p fills after 2s comes from Aufbau.
Once the 3d subshell has electrons in it, the 4s electrons end up higher in energy and farther out, so they leave first. That's why an ion with configuration [Ar]3d⁶ and a 2+ charge corresponds to neutral iron, [Ar]4s²3d⁶.
It's an excited state. An electron skipped a lower-energy spot (2s isn't full before 2p has an electron), which means the atom absorbed energy. The Aufbau principle only describes the ground state, the lowest-energy arrangement.