The number of unique open-chain structures corresponding to the molecular formula C₃H₅Cl is two.
<em>Step 1</em>. Draw <em>all possible connections of three carbon atoms</em>.
The is only one possibility (See first image below).
<em>Step 2</em>. Put the <em>Cl in all possible unique positions</em>.
There are only <em>two</em> possibilities: on an end carbon or on the middle carbon (see second image below).
Thus, there are only two isomers of C₃H₅Cl.