Compare Aufbau Principle with Hund's Rule and Pauli Principle

The Aufbau Principle

• What it says: Electrons fill the lowest-energy orbitals first before filling higher-energy orbitals.
• Bus Analogy: Passengers must fill the front row (lowest energy) before anyone can sit in the second row, and so on.
• Role: Determines the order of filling shells and subshells (e.g., 1s before 2s, 2s before 2p, 4s before 3d).

The Pauli Exclusion Principle

• What it says: No two electrons in an atom can have the same four quantum numbers. This practically means that an orbital can hold a maximum of two electrons, and they must have opposite spins (spin-up ↑ and spin-down ↓).
• Bus Analogy: Each seat (orbital) can hold a maximum of two people (electrons). If two people share a seat, they must be "opposite".
• Role: Determines the maximum capacity of any orbital.

Hund's Rule (of Maximum Multiplicity)

• What it says: When filling degenerate orbitals (orbitals of the same energy, like the three p-orbitals), electrons will first occupy each orbital singly with parallel spins (all "spin-up") before any orbital is doubly occupied.
• Bus Analogy: When passengers get to a row with multiple empty seats (like a p-subshell with 3 seats), they will each take their own empty seat first. Only after every seat in that row has one person will they start pairing up.
• Role: Determines the exact arrangement of electrons within a subshell (like p, d, f).