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Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. Geometric isomerism is sometimes considered synonymous with stereoisomerism, but is often used to describe just those stereoisomers that are not mirror-images of each other.
A stereoisomer generally occurs when a double or triple bond is present, because the pi pond involved prevents that bond from being "twisted" the same way that a single bond can be. One of the best examples is C2H2Cl2. Consider the two examples below:
H H H Cl
\ / \ /
C = C C = C
/ \ / \
Cl Cl Cl H
These are stereoisomeres because you cannot rotate the two carbon atoms relative to each other, due to the rigidity caused by the pi bond between them. Therefore one will not fit on top of the other.
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