Aufbau principle, Pauli exclusion principle and Hund's rule Transitional Reasoning Download PDF

Questions

1. 1.

   A student analyzes three proposed electron arrangements for the three $2p$ valence electrons of a neutral nitrogen atom ($Z = 7$). Arrangement I pairs two electrons with opposite spins in one $2p$ orbital and places one electron in a second $2p$ orbital, leaving the third empty. Arrangement II places one electron in each of the three degenerate $2p$ orbitals, but two electrons have spin up ($+\frac{1}{2}$) and one has spin down ($-\frac{1}{2}$). Arrangement III places two electrons with identical parallel spins ($+\frac{1}{2}$) in the same $2p$ orbital and one electron in an adjacent orbital. Which conclusion correctly evaluates the physical validity and energetic status of these arrangements?

   • A) Arrangement I violates the Pauli exclusion principle, whereas Arrangement III represents a permissible excited state.
   • B) Arrangements I and II represent permissible excited states that violate Hund's rule, whereas Arrangement III represents a physically impossible state that violates the Pauli exclusion principle.
   • C) Arrangement II violates the Aufbau principle because electrons must pair up before filling subsequent degenerate orbitals.
   • D) Arrangements I, II, and III are all physically impossible because degenerate orbitals must always possess strictly identical spin quantum numbers.