Isomers are molecules with the same chemical make-up but different strucural make-up. It's like a puzzle with the pieces but arranged differently. Glucose and fructose are isomers of each other. They both have the same chemical formula (C6H12O6), but yet they are arranged different structurally. Observe:
Glucose:
H   H   H   OH  H   H
|   |   |   |   |   |
H - C - C - C - C - C - C = 0
|   |   |   |   |
OH  OH  OH  H   OH
Fructose:
H   H   H   OH      H
|   |   |   |       |
H - C - C - C - C - C - C - OH
|   |   |   |   ||  |
OH  OH  OH  H   0   H
Notice how they are very similar - exactly the same amount of atoms, but the atoms are arranged differently. This is a small but crucial differrence. For example, they each taste quite different and interact differently with different enzymes, despite their chemical similarity. In laymen's terms, isomers are simply molecules with identical formulas but different shapes.
-- and | indicate single covalent bonds.
= and || inidicate double bonds.
I"so*mer, n. [See Isomeric.] Chem.
A body or compound which is isomeric with another body or compound; a member of an isomeric series.
Isomeric enantiomer Isomerism 3,4-METHYLENEDIOXY-2-METHYLTHIOAMPHETAMINE
glucose Chemistry constitutional isomer 6-(2-aminopropyl)-5-methoxy-2-methyl-2,3-dihydrobenzofuran
stereochemical configuration Amino acid Optical isomerism para-aminobenzoic acid
Racemic galactose polymer 2,5-Dimethoxy-4-(n)-Butylamphetamine
geriatric medicine gentiobiose Bolete Cisplatin
carvone cinnamaldehyde