Final answer:
A 'trans' geometrical isomer has similar substituents on the opposite side of a double bond, while a 'cis' isomer has similar substituents on the same side. This occurs due to the lack of free rotation around the carbon-carbon double bond in alkenes and cycloalkanes, resulting in distinct spatial arrangements for the substituents.
Step-by-step explanation:
Geometric isomers, such as cis and trans isomers in alkenes and cycloalkanes, differ in the spatial arrangement of atoms around a double bond or a ring structure. In the case of cis isomers, similar substituents are located on the same side of the double bond, leading to a change in the direction or a bend in the carbon backbone. Conversely, in the trans isomers, similar substituents are on opposite sides of the double bond, resulting in a more linear structure. An example of this can be seen in 2-butene, which can exist as cis-2-butene or trans-2-butene, where the two methyl groups or hydrogen atoms are on the same or opposite sides, respectively.
These isomers arise because there is no free rotation around the carbon-carbon double bond, due to the nature of the pi bond. Also, for geometric isomerism to occur, each of the carbon atoms involved in the double bond must have two different groups attached. This is why compounds like propene do not exhibit geometric isomerism since one of the carbons is attached to two hydrogen atoms.