1. Higher atomic number = higher priority
I > Br > Cl > S > O > N > C > H

2. If atoms are same, go to next atom outward

3. Double/triple bonds: "phantom atoms"
C=O counts as C bonded to (O,O,_)

4. Lowest priority (#4) pointing AWAY from you:
1→2→3 Clockwise = R (rectus)
1→2→3 Counter-CW = S (sinister)

n chiral centers:
Max stereoisomers = 2ⁿ

1 chiral center:
2 enantiomers (R and S)

2 chiral centers:
Up to 4 stereoisomers
(may have meso form)

Thalidomide tragedy: one enantiomer treats morning sickness, the other causes birth defects. Drug regulation now requires stereochemical purity.