Hund's rule is used to determine the ground state of which system?

Hund's rule is used to determine the ground state of which system?

• A. An atom
• B. A molecule
• C. A crystal lattice
• D. A plasma

Answer: (A)

Hund’s rule is about how electrons fill degenerate orbitals to determine the ground state of an atom. When several orbitals have the same energy (like the three 2p orbitals), the electrons first occupy different orbitals with their spins aligned (unpaired) before any pairing occurs. This arrangement maximizes the total spin, which lowers the energy thanks to exchange interactions and reduced electron–electron repulsion in the multi-electron wavefunction. A concrete example is carbon, where two electrons in the 2p subshell occupy two of the three degenerate p orbitals with parallel spins, giving a triplet ground state. In molecules, crystals, or plasmas, the overall energy landscape involves bonding, band structure, lattice effects, and collective behavior, so Hund’s rule doesn’t govern the ground state in the same direct way. Hence, Hund’s rule is used to determine the ground state of an atom.

Hund’s rule is about how electrons fill degenerate orbitals to determine the ground state of an atom. When several orbitals have the same energy (like the three 2p orbitals), the electrons first occupy different orbitals with their spins aligned (unpaired) before any pairing occurs. This arrangement maximizes the total spin, which lowers the energy thanks to exchange interactions and reduced electron–electron repulsion in the multi-electron wavefunction. A concrete example is carbon, where two electrons in the 2p subshell occupy two of the three degenerate p orbitals with parallel spins, giving a triplet ground state. In molecules, crystals, or plasmas, the overall energy landscape involves bonding, band structure, lattice effects, and collective behavior, so Hund’s rule doesn’t govern the ground state in the same direct way. Hence, Hund’s rule is used to determine the ground state of an atom.