Hund’s Rule
Hund’s Rule is one of the most important principles in atomic structure and electron configuration. It explains how electrons fill orbitals that have the same energy, known as degenerate orbitals. According to this rule, electrons prefer to occupy each orbital singly before pairing up. This simple but powerful concept helps scientists and students understand the behavior of atoms, how they bond, and why certain elements show magnetic properties.
Hund’s Rule works alongside the Aufbau Principle and the Pauli Exclusion Principle to describe the most stable arrangement of electrons in an atom. By applying this rule, we can accurately predict how electrons fill subshells in p, d, and f orbitals, and understand the chemical and magnetic nature of elements.
What is Hund’s Rule?
Hund’s Rule is one of the fundamental rules in chemistry that helps us understand how electrons are arranged inside atoms. It is especially important when electrons are placed into orbitals that have the same energy level, known as degenerate orbitals. According to Hund’s Rule, when electrons fill orbitals of equal energy, they first occupy each orbital singly before pairing up. This rule helps atoms achieve the most stable and lowest energy configuration.
Explanation of Hund’s Rule
To understand Hund’s Rule, we need to remember that electrons are negatively charged and naturally repel each other. To reduce this repulsion and make the atom more stable, electrons prefer to stay alone in an orbital before joining another electron in the same orbital.
Imagine a bus with empty seats. Just like passengers prefer sitting alone in a seat before sitting next to someone, electrons also prefer to stay in different orbitals first. Only after each orbital of the same energy level has one electron, do the electrons start to pair up in those orbitals.
This behavior is most common in p, d, and f orbitals where there are multiple orbitals within the same subshell. Hund’s Rule works along with the Aufbau Principle (electrons fill the lowest energy orbital first) and the Pauli Exclusion Principle (no two electrons in the same orbital can have the same spin).
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Do Check: Acetaldehyde
Order of Electron Filling in Degenerate Orbitals
Here’s how electrons fill degenerate orbitals according to Hund’s Rule:
| Step | Action | Explanation |
| 1 | Each orbital in a subshell gets one electron | Electrons occupy empty orbitals first |
| 2 | Electrons in these orbitals have the same spin | Usually represented by an upward arrow (↑) |
| 3 | After each orbital has one electron, pairing begins | The second electron in an orbital has an opposite spin (↓) |
This filling pattern helps to minimize electron-electron repulsion and ensures maximum stability for the atom.
Do Check: Isomeris
Examples of Hund’s Rule
Let’s look at some common examples where Hund’s Rule is applied to electron configurations:
| Element | Electron Configuration | 2p/3d Orbital Representation | Explanation |
| Carbon (C) | 1s² 2s² 2p² | ↑ ↑ _ | Two electrons in separate 2p orbitals, unpaired |
| Nitrogen (N) | 1s² 2s² 2p³ | ↑ ↑ ↑ | Three unpaired electrons, each in its own orbital |
| Oxygen (O) | 1s² 2s² 2p⁴ | ↑↓ ↑ ↑ | Two electrons are paired in one orbital, rest unpaired |
| Iron (Fe) | [Ar] 4s² 3d⁶ | ↑↓ ↑ ↑ ↑ ↑ | d-orbitals filled singly first, then pairing starts |
As seen in these examples, electrons avoid pairing up until each orbital of the same energy level has one electron.
Importance of Hund’s Rule
Hund’s Rule is important for several reasons in chemistry and physics:
1. Determines Correct Electron Configuration
Hund’s Rule ensures that we correctly predict how electrons are arranged in atoms. Without it, our understanding of the atom’s structure would be incomplete or incorrect.
2. Explains Magnetic Properties
Atoms with unpaired electrons show paramagnetism (they are attracted to a magnetic field). Atoms with all paired electrons are diamagnetic (they are slightly repelled by a magnetic field). This is essential in understanding the behavior of elements in magnets and various chemical reactions.
3. Influences Chemical Bonding
Hund’s Rule also affects how atoms form chemical bonds. Unpaired electrons in degenerate orbitals are often available for bonding, which determines how atoms interact with other atoms.
4. Predicts Reactivity of Elements
Elements with partially filled orbitals are often more reactive. For example, oxygen and nitrogen have unpaired electrons and are chemically very active. This is useful in predicting how a substance might behave in a chemical reaction.
Do Check: Reactivity Series
Conclusion
Hund’s Rule is a key rule in chemistry that tells us how electrons fill orbitals of the same energy. It ensures that electrons spread out before they pair up, reducing repulsion and making atoms more stable. This rule plays a crucial role in understanding magnetism, bonding, electron configuration, and chemical reactivity. Whether you're a student or a curious learner, mastering Hund’s Rule helps you build a strong foundation in atomic structure and quantum chemistry.
Hund’s Rule FAQs
What is Hund’s Rule in simple words?
Hund’s Rule states that electrons fill orbitals of the same energy one at a time before they start pairing. This helps minimize repulsion between electrons and keeps the atom more stable.
Why do electrons follow Hund’s Rule?
Electrons follow Hund’s Rule to reduce repulsion and achieve the lowest possible energy configuration. This makes the atom more stable and balanced.
Which orbitals does Hund’s Rule apply to?
Hund’s Rule applies to degenerate orbitals—those with the same energy level—like p, d, and f orbitals. It does not apply to the s orbital since it has only one sub-orbital.
How does Hund’s Rule affect magnetism?
Elements with unpaired electrons due to Hund’s Rule show paramagnetism (they are attracted to magnetic fields). Atoms with only paired electrons are diamagnetic (repelled by magnetic fields).
Can Hund’s Rule be broken or violated?
In normal conditions, atoms follow Hund’s Rule. However, under special conditions like strong electric or magnetic fields, electron arrangements can change temporarily, leading to exceptions.