--- title: "Hund's Rule in College Physics I" description: "Hund's Rule says electrons fill degenerate orbitals singly before pairing, lowering repulsion and shaping electron configurations in College Physics I." canonical: "https://fiveable.me/intro-college-physics/key-terms/hunds-rule" type: "key-term" subject: "College Physics I – Introduction" unit: "Unit 30" --- # Hund's Rule in College Physics I ## Definition Hund's Rule says electrons occupy degenerate orbitals one at a time before any pairing happens. In College Physics I, it helps you build the lowest-energy electron configuration for atoms. ## What It Is Hund's Rule is the rule you use when an atom has several orbitals at the same energy level, called degenerate orbitals. In College Physics I, it tells you to place one electron in each of those orbitals before you start pairing electrons in the same orbital. That pattern is not random. Electrons are negatively charged, so they repel each other. If you spread them out across equal-energy orbitals first, the atom stays a little lower in energy than it would if you forced two electrons into the same orbital too soon. That lower-energy arrangement is usually the ground-state arrangement, which is the one you want when writing electron configurations. This rule comes up most clearly in subshells with multiple orbitals, like the p, d, and f subshells. A p subshell has three orbitals, so three electrons will occupy them separately before any pairing starts. That is why nitrogen, with three electrons in its 2p subshell, is written with three unpaired electrons in three different p orbitals. Oxygen comes right after that, so its fourth 2p electron has to pair up in one of those orbitals. Hund's Rule works together with the Pauli Exclusion Principle, but they are not the same thing. Pauli says that two electrons in the same orbital must have opposite spins. Hund's Rule says you do not jump to pairing until you have filled the degenerate orbitals singly first. One rule controls how electrons can share an orbital, and the other controls the order in which they enter orbitals of equal energy. A useful way to picture it is with orbital box diagrams. If you have three p orbitals and three electrons, the boxes each get one arrow before any box gets two. If you skip that step and pair too early, you will still violate the ground-state pattern even if you do not violate Pauli. That is why a configuration can be allowed by spin rules but still be the wrong lowest-energy arrangement. ## Why It Matters Hund's Rule matters because electron arrangement is the starting point for a lot of atomic behavior in this course. When you know how electrons fill orbitals, you can predict which electrons are unpaired, which orbitals are half-filled, and how much energy it takes to add or rearrange electrons. That shows up in diagrams, electron configurations, and questions about magnetic behavior. Atoms with unpaired electrons are often paramagnetic, while atoms with all electrons paired are diamagnetic. So Hund's Rule is not just a bookkeeping trick, it helps connect orbital filling to measurable properties. It also helps you avoid common configuration mistakes. Many students can count electrons correctly but still place them incorrectly inside the subshell boxes. If you know the rule, you can check whether a drawing matches the lowest-energy state instead of just whether the total number of electrons is right. In the bigger quantum model, Hund's Rule sits between the quantum numbers and the Pauli Exclusion Principle. Quantum numbers tell you where an electron can go, Pauli limits how two electrons can share a single orbital, and Hund's Rule tells you how to spread electrons across equal-energy orbitals first. That sequence is a big part of how atomic structure is organized in College Physics I. ## Connections ### Degenerate Orbitals Hund's Rule only applies when orbitals have the same energy, which is what degenerate means. In a p, d, or f subshell, the orbitals are degenerate before outside forces split their energies. If the orbitals are not degenerate, Hund's Rule is not the step you use to decide electron placement. ### Pauli Exclusion Principle Pauli and Hund's Rule work together, but they answer different questions. Pauli says no two electrons in one orbital can have the same set of quantum numbers, so paired electrons must have opposite spin. Hund's Rule decides when pairing happens, which is after each degenerate orbital already has one electron. ### Quantum Numbers Quantum numbers describe the allowed states for electrons, and Hund's Rule helps you choose the lowest-energy arrangement among those allowed states. The principal and magnetic quantum numbers help locate the electron in a shell and orbital, while Hund's Rule guides how electrons fill orbitals within the same subshell. ### [Magnetic Quantum Number](/intro-college-physics/key-terms/magnetic-quantum-number) The magnetic quantum number identifies which orbital inside a subshell an electron occupies. Hund's Rule uses that orbital-by-orbital structure directly, because it tells you to spread electrons across different magnetic quantum number values before pairing them in one orbital. ## On the AP Exam A quiz question or problem set item will usually ask you to draw an orbital box diagram, write an electron configuration, or identify the ground-state arrangement for a specific atom. That is where Hund's Rule shows up most directly. You use it by filling each degenerate orbital with one electron first, then pairing only after every orbital in that subshell has one electron. If you are given a configuration and asked whether it is correct, check two things: the total number of electrons and the filling order inside each subshell. A wrong answer often has the right total count but pairs electrons too early in a p or d subshell. If the course includes short written responses, you may also explain why the arrangement is lower in energy because it reduces electron-electron repulsion. ## Hund's Rule vs Pauli Exclusion Principle Hund's Rule and the Pauli Exclusion Principle both affect how electrons are arranged, but they do different jobs. Hund's Rule says electrons spread out into separate degenerate orbitals before pairing. Pauli says two electrons can share one orbital only if they have opposite spins. If you mix them up, you may know that electrons cannot be identical in one orbital, but still place them in the wrong order across the subshell. ## Key Takeaways - Hund's Rule says electrons fill degenerate orbitals one at a time before any pairing starts. - The rule lowers energy by reducing electron-electron repulsion in atoms. - You see Hund's Rule most clearly in p, d, and f subshell orbital box diagrams. - It works with, but is not the same as, the Pauli Exclusion Principle. - If a configuration pairs electrons too early, it is not the correct ground-state arrangement. ## FAQs ### What is Hund's Rule in College Physics I? Hund's Rule says that when electrons enter degenerate orbitals, they occupy each orbital singly before they begin to pair up. In College Physics I, you use it to build the lowest-energy electron configuration for an atom. ### How is Hund's Rule different from the Pauli Exclusion Principle? Hund's Rule decides the order electrons fill equal-energy orbitals, while the Pauli Exclusion Principle limits how two electrons can share one orbital. Pauli says paired electrons must have opposite spins, but Hund's Rule says you do not pair them until you have to. ### What is an example of Hund's Rule? A p subshell has three orbitals. If there are three electrons available, Hund's Rule puts one electron in each p orbital instead of pairing any of them early. Nitrogen is a common example because its 2p electrons are spread across three orbitals. ### Why does Hund's Rule matter for electron configurations? It helps you choose the ground-state arrangement, not just the correct electron count. If you ignore it, you might still put the right number of electrons in a subshell but show the wrong orbital diagram and the wrong energy arrangement. ## Related Study Guides - [30.9 The Pauli Exclusion Principle](/intro-college-physics/unit-30/9-pauli-exclusion-principle/study-guide/aWwJyTeu9NHe2308) - [30.8 Quantum Numbers and Rules](/intro-college-physics/unit-30/8-quantum-numbers-rules/study-guide/qKgARBjiujCZQZpo) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/intro-college-physics/key-terms/hunds-rule#resource","name":"Hund's Rule in College Physics I","url":"https://fiveable.me/intro-college-physics/key-terms/hunds-rule","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/intro-college-physics/key-terms/hunds-rule#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:22:15.477Z","isPartOf":{"@type":"Collection","name":"College Physics I – Introduction Key Terms","url":"https://fiveable.me/intro-college-physics/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/intro-college-physics/key-terms/hunds-rule#term","name":"Hund's Rule","description":"Hund's Rule says electrons occupy degenerate orbitals one at a time before any pairing happens. 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