 Hund's rule of maximum multiplicity states that for orbitals of equal energy (degenerate orbitals), electrons:
APair up in one orbital first before going to others
BAlways pair up with opposite spins immediately
CSingly occupy each orbital with parallel spin before pairing
DOccupy orbitals based on alphabetical labels
Answer & Solution
Correct answer: C. Singly occupy each orbital with parallel spin before pairing
Hund's rule of maximum multiplicity: among degenerate orbitals (e.g. the three p-orbitals), electrons enter singly with parallel spins first; pairing begins only after each orbital has one electron.
Reason: pairing costs energy due to electron-electron repulsion. The half-filled state with parallel spins is more stable (exchange energy stabilisation), which is why a half-filled p, d, f subshell has extra stability (think Cr and Cu electron configurations).
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