Explain why the compounds of the type `C_2 H_2 X_2` exhibit cis-trans isomerism.

Explanation

The compounds of the type `C_2 H_2 X_2` possess a C=C double bond. Atoms cannot rotate around a double bond. Therefore, following two orientations of atoms in space are possible
`{:(H-C-X" "H-C-X),(" "||" "||),(H-C-X " "X-C-H),(" cis" " trans"):}`
The isomer in which similar atoms are on the same side of double bond is called a cis isomer while that in which similar atoms are on the opposite sides is called a trans isomer and the compound is said to show cis-trans isomerism.