Hund's rule, in the context of Valence Bond Theory (VBT), states that for a given electronic configuration, the term with the maximum multiplicity has the lowest energy. Essentially, electrons will individually occupy each orbital within a subshell before any orbital is doubly occupied.
Here's a breakdown:
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Maximum Multiplicity: Multiplicity refers to the number of unpaired electrons plus one (2S+1, where S is the total spin angular momentum). Hund's rule dictates that electron configurations with more unpaired electrons (higher multiplicity) are more stable.
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Valence Bond Theory (VBT) Context: While Hund's rule is a general principle in atomic and molecular electronic structure, its application within VBT emphasizes how electron pairing influences the stability of chemical bonds. VBT focuses on the formation of bonds through the overlap of atomic orbitals, and Hund's rule helps determine the most favorable electronic arrangement for this overlap.
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Why does this happen? This rule is based on two main factors:
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Exchange Energy: When electrons with the same spin are in different orbitals, they can exchange places without changing the overall energy of the system. This exchange is a stabilizing interaction, and the greater the number of possible exchanges (more unpaired electrons), the lower the energy.
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Electron Repulsion: Minimizing electron-electron repulsion is also a driving force. By occupying different orbitals, electrons can spread out and minimize their mutual repulsion, which lowers the overall energy.
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Practical Implications & Examples:
Consider filling the 2p orbitals, which have three degenerate orbitals (2px, 2py, 2pz).
- Carbon (C): Carbon has 6 electrons. According to Hund's rule, its electronic configuration is 1s² 2s² 2p¹x 2p¹y 2p⁰z, meaning each of the first two 2p orbitals are singly occupied before either is doubly occupied. This gives carbon two unpaired electrons, making it divalent.
- Nitrogen (N): Nitrogen has 7 electrons. Following Hund's rule, its electronic configuration is 1s² 2s² 2p¹x 2p¹y 2p¹z. All three 2p orbitals are singly occupied with parallel spins. This arrangement gives nitrogen three unpaired electrons, making it trivalent and particularly stable.
- Oxygen (O): Oxygen has 8 electrons. Its electronic configuration is 1s² 2s² 2p²x 2p¹y 2p¹z. Now, one of the 2p orbitals has to have two electrons, minimizing the exchange energy compared to forcing two electrons into another unfilled p orbital, but is still maximizes multiplicity before completely filling orbitals.
In Summary:
Hund's rule ensures that electrons maximize their unpaired spin within a subshell before pairing up. This maximizes exchange energy and minimizes electron-electron repulsion, leading to a more stable electronic configuration, which is crucial to consider in Valence Bond Theory when determining bonding arrangements and molecular properties.
