With stero isomerism the molecule have the same molecular formula, and the same connections between the atoms, but different arrangements of the atoms
Again there are two different types of stereo isomerism. These are:
Optical Isomerism
and
Geometeric Isomerism
Optical isomerism is more commonly known as chiral isomers, or as chirality.
Optical isomers vary in their three dimentisional space. Because of this they are, actually, non-superimpossible mirror images. A good way to understand this is by looking closely at your hands, they have very similar structures, Yet you can see that hands are not identical, and that they will not fit on top of each other (when both facing up), they can not be superimposed on top of each other. And you can see that they are mirror images of each other. Optical isomerism is the same type of thing the two molecules mirror each other.
They are NOT the same compound!
Organic molecules are considered chiral if they contain one or more chiral centres. The chiral centre is the atom which is central of both of the none superimpossible compounds. Often Organic compounds are easily identified because the chiral centre is an sp3 hybridized carbon that is bonded to four different groups.
Inorganic molecules are more complicated, the best way to identify a chiral inorganic molecule is by making a model of it or by imaging how its mirror image would look.
With Chirality there can only be two isomers per chiral centre. These two compounds are called enantiomers. (They are nonsuperimposable mirror images). Enantiomers can be told a part using plan polarised light, as when both enantiomers are pressent in equal amount there will be no over all rotation of the light, but when seperate, one enantiomer will rotate the light anti clockwise and the other will rotate the light clockwise.
The diagrams above show both the enantiomers of 2-hydroxy-but-3-ene. As you can see they look very simular in structure, yet you can also tel if you look very closely that they can not be superimposed upon each other, and that they are mirror images of each other.
Geometric isomerism is also know (usually in america) as cis/trans isomerism. This type of isomerism is usually seen in molecules which either are square planner or have a double bond. (see below for examples)
With Geometric isomerism the isomers are named after their shape. For example, the cis isomer will be called cis-... what ever the name of it is, and the trans isomer will be called trans-... what ever the name of the isomer is.
With the cis isomers the two functional groups (which are the same molecule) are next to each other, so to put it simply cis means next to.
With the trans isomers the two functional groups (which are the same molecule) are opposit each other, so to put it simply trans means opposit.
The above diagram shows the cis and trans platin. The cis platin is the platin in which the Cl molecule is next to the other Cl molecule and one of the NH3 molecules and is opposit to the other NH3 molecule. The trans platin is the molecule in which the Cl molecules are opposit to each other, and next to both the NH3 molecules which are also opposit each other.
The above diagrams show the cis and trans isomers of but-2-ene. Again as you can see the with the cis isomer the to methyl groups are on the same side as each other. And with the trans isomer the to methyl groups are opposit each other.
Author: Catherine Spowage (document modification date: 21st may 2003)