Rules for Filling Electrons in Orbitals

1. Aufbau Principle  

- Electrons fill orbitals in order of increasing energy levels.  

- Lower‑energy orbitals are filled before higher‑energy ones.  

2. Hund’s Rule  

- Electrons occupy orbitals of the same subshell singly first (with parallel spins).  

- Pairing occurs only after each orbital has one electron.  

3. Pauli’s Exclusion Principle  

- No two electrons in the same atom can have identical quantum numbers.  

- Each orbital can hold a maximum of two electrons with opposite spins.