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Atomic Structure and Electron Configuration

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An atom's chemistry is set by how its electrons are arranged. Electrons occupy orbitals grouped into shells and subshells, and they fill from lowest energy upward, following the Aufbau order, Hund's rule, and the Pauli principle. The resulting electron configuration is the fingerprint that drives an element's bonding and its place in the periodic table.

UNIT 1 TOPIC 1.5 • ATOMIC STRUCTURE AND ELECTRON CONFIGURATION ORBITAL FILLER PHOSPHORUS (P) proton neutron e⁻ ELECTRON SHELLS 2 n=1 8 n=2 5 n=3 2 – 8 – 5 15 P Phosphorus 30.97 ORBITAL FILLING (AUFBAU ORDER) 1s 2s 2p 3s 3p 3 UNPAIRED e⁻ (HUND) VALENCE e⁻ = 5 ELECTRON CONFIGURATION 1s² 2s² 2p⁶ 3s² 3p³ shells 2-8-5 AUFBAU PRINCIPLE Electrons fill the lowest- energy orbitals first. PAULI EXCLUSION Max 2 e⁻ per orbital, held with opposite (paired) spins. HUND'S RULE Singly fill degenerate orbitals before pairing (parallel spins). COULOMB'S LAW Attraction sets orbital energies: e⁻ closer / lower-n orbitals are held more tightly. F ∝ q₁q₂ r² CED ANCHOR Topic 1.5: electron configuration follows Aufbau, Pauli & Hund — verify p-subshells. AP Chemistry · Unit 1 · Atomic Structure & Properties
Phosphorus, built electron by electron. Fifteen electrons fill from the lowest-energy orbitals outward, giving the 2-8-5 shell picture and the configuration 1s² 2s² 2p⁶ 3s² 3p³. Filling order follows energy, not simply shell number.
Orbital Filler · Open the sandbox →

The recurring errors are about energy order, not counting: assuming a higher shell number always means higher energy, pairing electrons in a subshell before each orbital has one, and removing electrons for an ion in the reverse of the order they went in. Electrons leave from the highest occupied shell, which is not always the last one filled.

The work

3 ways in · any order
Lesson
Electron Configuration ›

Electron configurations follow filling rules that trip students when energy order and shell number disagree. The lesson works the Aufbau, Hund, and Pauli rules, then closes with a ten-scenario applet that fills orbitals and forms ions.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the three Topic 1.5 misconceptions: reading shell number as energy order, pairing electrons before every orbital is half-filled, and removing ion electrons last-in-first-out instead of from the highest shell.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
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