Electron Rules
Unit 4 – The Electron
Electron Rules
Where do electrons go?
There are 3 rules that govern…
Electron Rules1. Aufbau Principle
• Electrons enter the lowest possible energy levels first
• Start with PEL• PEL 1 has the lowest E, then 2,etc
• Then sublevels within a PEL• S is the lowest then p,d,f
• Each orbital in a sublevel is equal in E
Electron Rules
BUT E- from very full PELs can spill into other
PELs Sometimes it takes less E to go to the next
PEL
Electron Rules
Order of orbital filling – follow the arrow
1s2s 2p3s 3p 3d4s 4p 4d 4f5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f
Electron Rules
2. Pauli Exclusion Principle
• Maximum of 2e- per orbital• Must have
opposite ‘spins’
Electron Rules3. Hund’s Rule• In orbitals of equal E, e- fill orbitals one-by-
one until they HAVE TO pair up!• Orbitals take 1 e- each with parallel spins,
before doubling up
• Example:2p with 2e-
Spread the electrons outWith parallel spins as much as you
Can before pairing
Electron Rules
Orbital Diagrams Visual representation of electron orbital
filling Examples: H
1s
Electron Rules
Examples: He
Li
C
1s
1s 2s
2s 2p1s
Electron Rules
Electron Configurations Shorthand version of orbital diagrams
1s2Principal Energy Level
Sublevel (orbital type)
# of e- in sublevel
Electron Rules Examples: H 1s1
He 1s2
Li 1s22s1
C 1s22s22p2
Electron Rules Noble Gas Abbreviations
Shorthand version of electron configurations Example: Ne electron configuration
1s22s22p6
Na electron configuration: 1s22s22p63s1
Na with Noble Gas Abbreviation: [Ne] 3s1