ELECTRON CONFIGURATION
3 Rules:
Aufbau: fill lowest energy first
Pauli: max 2e⁻ per orbital (opp. spins)
Hund: 1e⁻ per orbital before pairing
s subshell
1 orbital max 2e⁻
p subshell
3 orbitals max 6e⁻
d subshell
5 orbitals max 10e⁻
f subshell
7 orbitals max 14e⁻
Fill order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p… (4s before 3d!)
EXAMPLES
Na (11)
1s²2s²2p⁶3s¹ = [Ne]3s¹
PERIODIC TRENDS
Why trends exist: nuclear charge pulls e⁻ in; shielding by inner e⁻ pushes them out. Across a period: nuclear charge wins. Down a group: shielding wins.
ATOMIC RADIUS
↓ group: increases (more shells added)
→ period: decreases (more p⁺, same shell, stronger pull)
IONIZATION ENERGY (remove 1 e⁻)
↓ group: decreases (e⁻ farther out, easier to remove)
→ period: increases (tighter nuclear grip)
ELECTRONEGATIVITY (attract bonding e⁻)
↓ group: decreases
→ period: increases
Highest: F = 4.0 · Lowest: Fr ≈ 0.7
PERIODIC TABLE LAYOUT
Period (row)
= # electron shells
Group (col)
= # valence e⁻
Alkali metals
Group 1 very reactive
Alkaline earth
Group 2 reactive metals
Halogens
Group 17 reactive NM
Noble gases
Group 18 inert