AP Chemistry · Unit 1 · Topic 1.5
Atomic Structure and Electron Configuration
An electron configuration is a compact model of where an atom's electrons can be found energetically. It is not a photograph of little planets traveling along fixed tracks. Read the notation as a set of occupied orbitals, then ask which electrons are available to influence chemical behavior.
Count particles before filling orbitals
Atomic number fixes the number of protons. A neutral atom has the same number of electrons; a positive ion has fewer and a negative ion has more. Isotope changes alter neutron count without changing a neutral atom's electron count. For example, sulfur has 16 electrons as a neutral atom and 18 in S2−, regardless of which sulfur isotope is being considered.
Three rules and what they mean
Use the usual ground-state filling order as a starting model. An orbital holds at most two electrons with opposite spins. Within a set of equal-energy orbitals, place electrons singly with parallel spins before pairing. This is Hund's rule, and it concerns orbitals within the same subshell, not a license to skip into a higher-energy shell.
Decode the symbols
| Subshell | Number of orbitals | Maximum electrons | Example notation |
|---|---|---|---|
| s | 1 | 2 | 2s² |
| p | 3 | 6 | 2p⁴ |
| d | 5 | 10 | 3d⁶ |
The leading number identifies the principal shell, the letter the subshell type, and the superscript the number of electrons occupying that subshell. A p superscript of four is allowed; a single p orbital containing four electrons is not.
Worked example: sulfur and sulfide
For neutral sulfur, distribute 16 electrons as 1s² 2s² 2p⁶ 3s² 3p⁴, or [Ne]3s²3p⁴. In the three 3p orbitals, one pair and two single electrons give two unpaired electrons. Adding two electrons produces S2− with [Ne]3s²3p⁶, the same electron count as argon. The proton count stays at 16, so the ion has not become an argon atom.
Ion formation needs an extra check
Main-group cations generally lose their outermost electrons first. For transition-metal ions, remove electrons from the highest principal shell before the remaining d electrons: Fe is commonly written [Ar]4s²3d⁶, while Fe2+ is [Ar]3d⁶. Neutral-atom filling order is not simply reversed without considering the changed orbital energies in the ion.
Use observations to test the model
A species with unpaired electrons is paramagnetic in this introductory description and is attracted to a magnetic field. The predicted two unpaired electrons in an isolated sulfur atom therefore provide a testable consequence of its configuration. Spectroscopic evidence also supports discrete subshell energies. A configuration should explain observations, not merely satisfy a memorized sequence.
Model boundaries and frequent mistakes
Some neutral atoms, including chromium and copper, depart from the simplest filling pattern; use established configurations when required. Do not describe an orbital as a sharply bounded container or the electron's exact trajectory. Finally, core electrons still affect shielding even though valence electrons usually dominate main-group bonding discussions.
Practice, then check your reasoning
How many electrons and unpaired electrons does neutral phosphorus have?
Answer and reasoning: 15 electrons; [Ne]3s²3p³ has three singly occupied 3p orbitals, so three are unpaired.
Does Cl− have the same nucleus as Ar?
Answer and reasoning: No. Both have 18 electrons, but chlorine has 17 protons and argon has 18.
Scientific references: OpenStax: electron configurations; OpenStax: atoms and ions. OpenStax Chemistry 2e authors: Paul Flowers, Klaus Theopold, Richard Langley, and William R. Robinson. Consult source pages for their license terms. Accessed September 13, 2026. Explanations, comparison tables, and practice tasks were independently written for this site; numerical teaching datasets are illustrative unless explicitly identified otherwise.
Official curriculum reference: College Board AP Chemistry course page. The explanation and worked example are independently written for this study site.