Isomers

• Isomers can form in a number of different ways. Some isomers are due to different connections between the same atoms, resulting in different chemicals. For example, dimethyl ether (CH3OCH3) and ethyl alcohol (CH3CH2OH) have the same atoms and yet are different compounds. These are structural isomers, since their structures differ. Cis and trans isomers are in a different class: the geometric isomers. These isomers have the same connections among atoms but are still different from one another. (ref 1)
  
  

Cis/Trans Double Bond Isomers

• When two carbon atoms are joined together in a molecule with a double bond, they cannot turn relative to one another. In some molecules, each of these carbons will have two different groups of atoms, A and B, attached and projecting away from it at an angle; one group up, one down. If the A group on one carbon projects upward and so does the A group on the other carbon, this is known as the cis isomer. If both groups project in opposite directions, you have the trans isomer. Since the double bond "locks" the carbons into position relative to each other, the cis and trans isomers cannot inter-convert and are distinct. (ref 1)
  
  

Cis/Trans Ring Isomers

• It's also possible to have cis and trans isomers when a series of carbon atoms are joined to one another in a cyclic manner to form a ring. In this case, carbons cannot rotate relative to each other because the ring structure prevents it. If two carbons in the ring have A and B groups, the cis isomer is the one in which both A groups are on the same side of the plane formed by the ring. The trans isomer is the compound in which one A group is above the ring, the other below. (ref 2)
  
  

Inorganic Cis/Trans Isomers

• Inorganic chemicals -- those where carbon is not the main element -- also have their own kind of cis and trans isomers. These occur in what are known as coordination compounds, in which there is a central metal atom surrounded by other atoms. In some of these compounds, two of the attached atoms will be the same. The cis isomer of these chemicals will be the one in which the two, identical atoms are adjacent to one another, while in the trans isomer, the identical atoms are separated. (ref 3)