... the parts is the same, but their 3-D orientation is different. One type of stereoisomerism common to compounds with C=C bonds is called cis/trans isomerism. Cis/trans isomerism occurs because a double bond prevents the internal rotation that would ordinarily occur if the double bond was a single bond. The atoms in a molecule are not static objects that are fixed in space, rather, they are constantly engaged in all sorts of internal motions, such as stretching and bending of bonds and rotation of parts of the molecule around different bonds. To picture internal rotation about a bond, think about two apples impaled on either end of a pencil. Both apples can easily be rotated perpendicular to the pencil by twisting them; atoms bonded together by a single bond are free to rotate in the same manner. If you added a second pencil between the apples, they would no longer be ...

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