A chiral (asymmetric) carbon has four different substituent groups arranged at the corners of a tetrahedron (sp³, ~109.5° bond angles). Its mirror image — the enantiomer — has the identical connectivity and bond lengths but opposite spatial arrangement, like a left hand and a right hand. No rotation in 3D space can turn one into the other: that is the geometric definition of chirality.
CIP priority (Cahn–Ingold–Prelog): rank the four substituents by the atomic number of the atom directly attached (ties broken by the next atoms out). Orient the molecule so the lowest-priority group (4) points away from you, then read the remaining three from highest to lowest priority (1→2→3). Clockwise = R (rectus), counter-clockwise = S (sinister). The curved arrow drawn between the colored groups shows exactly this 1→2→3 path.
Try the overlay: switch to "Overlay", then drag the translucent enantiomer to line its groups up with the opaque original. You can always get three of the four substituents to match — but the fourth will always be wrong, no matter how you rotate. That is a direct, hands-on proof of non-superimposability.
- Thalidomide — historically, one enantiomer acted as a sedative while its mirror image was teratogenic, causing severe birth defects in the late 1950s–60s. It taught pharmacology that the two mirror forms of a drug can have completely different biological effects.
- Limonene — R-limonene smells of orange/citrus, S-limonene smells of pine/turpentine. Same atoms, same bonds, opposite handedness — detected because our olfactory receptors are themselves chiral.
Simplified schematic: only the chiral center and its four substituent groups are modeled, not the full ring systems.