1. Chapter 13 Section 2 electron configuration- how e- are arranged around the nucleus Rules for e- configs: 1)Aufbau Principle -electrons enter orbitals of lower energy first s → p → d → f -atomic orbitals are represented as boxes s = 1 box (1 orbital) p = 3 boxes (3 orbitals) d = 5 boxes (5 orbitals) f = 7 boxes (7 orbitals)

2. 2)Pauli-Exclusion Principle -an atomic orbital can hold at most 2 e- -electrons are represented as arrows -spins are opposite -first electron is ↑ -second electron is ↓ -number of e- must equal number of arrows

3. 3)Hund’s Rule -one electron enters each orbital of equal energy until orbitals contain one electron, then they can hold two e- -it is more stable to have partially filled orbitals than empty orbitals