Hund's Rule and Aufbau Principle

Pauli Exclusion Principle

No two electrons in the same atom can have identical values for all four of their quantum numbers.

In other words:

(1) no more than two electrons can occupy the same orbital

(2) two electrons in the same orbital must have opposite spins 

Aufbau Principle

states that electrons fill lower-energy atomic orbitals before filling higher-energy ones (Aufbau is German for "building-up")

Hunds Rule

every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.

How to Write an Electron Configuration:

1. Start with the shell/period number

2. Followed by the type of orbital

3. Finally the superscript indicates the number of electrons in the orbital

Example: B = 1s² 2s² 2p¹