Molecule Relationships

Different Formulas: Different molecular formulas — not isomers at all.
Structural Isomers: Same molecular formula, but different connectivity of atoms.
Resonance Forms: Same connectivity and same molecular formula, but different arrangements of electrons (bonds and lone pairs).
Enantiomers: Non-superimposable mirror images. All stereocenters have opposite configurations (R↔S).
Diastereomers: Stereoisomers that are not mirror images. At least one (but not all) stereocenters differ.
Different Conformations: Same molecule in different rotational arrangements around single bonds (rotamers).
Identical: The same molecule drawn in a different orientation or style.

Quick Decision Guide

Step 1: Check molecular formula. Different formula → different molecules.
Step 2: Check connectivity. Different connectivity → structural isomers.
Step 3: Same connectivity? Check bond orders. Different bond arrangement → resonance forms.
Step 4: Same bonds? Check 3D arrangement. Rotamers → conformations.
Step 5: Check stereocenters. All same → identical. All opposite → enantiomers. Mixed → diastereomers.

Identify the relationship between Drawings A and B.

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